Method of detecting the transition, between a first compound and a second compound, of a product, which may contain such a first compound and/or such a second compound, and which circulates inside a conduit for conveying this product, characterised by the fact that: - an absorption spectrum is recorded for each sample of a plurality of samples of the first pure compound and for each sample of a plurality of samples of the second pure compound; - from the absorption spectra recorded for the samples of the first and second pure compounds, at least three spectral markers, common to the first compound and to the second compound, discriminating for these first and second compounds, and each constituted by a function of a plurality of absorptions of such a spectrum, are determined by statistical analysis of the absorption spectra by carrying out either a discriminant factorial analysis or a principal component analysis of these absorption spectra, without resorting to a characterisation based solely on physical properties relating to the compounds; - from the absorption spectra recorded for the samples of the first and second pure compounds, it is defined, on the one hand, a first spectral box made up of a first spectral zone delimited by values of at least these three determined spectral markers, and corresponding to the first pure component, and on the other hand, a second spectral box made up of a second spectral zone delimited by values of at least these three spectral markers, which were determined and that correspond to the second pure compound, at least for those three previously determined spectral markers and in a space corresponding to those spectral markers; - inside the conduit for conveyance of the product, at least one absorbance spectrum of this product circulating inside this conduit is recorded; - for at least one such absorption spectrum recorded for the product circulating inside the conduit, the value of at least three spectral markers corresponding to at least these three spectral markers previously determined for the first and second pure compounds is determined; - on a graph, the spectral boxes corresponding to at least one of the three spectral markers are represented, preferably at least the three spectral markers, for the first and second pure compounds; - on the same graph, the value of the spectral marker (s) determined and corresponding to at least one absorption spectrum recorded for the product circulating inside the conduit for conveyance is displayed; - the transition between the first compound and the second compound is detected for this product when the value of the spectral marker(s) determined and corresponding to this product moves away from the first spectral box and/or approaches the second spectral box, or even corresponds to this second spectral box. - an absorption spectrum is recorded for each sample of a plurality of samples of the first pure compound and for each sample of a plurality of samples of the second pure compound; - from the absorption spectra recorded for the samples of the first and second pure compounds, at least three spectral markers, common to the first compound and to the second compound, discriminating for these first and second compounds, and each constituted by a function of a plurality of absorptions of such a spectrum, are determined by statistical analysis of the absorption spectra by carrying out either a discriminant factorial analysis or a principal component analysis of these absorption spectra, without resorting to a characterisation based solely on physical properties relating to the compounds; - from the absorption spectra recorded for the samples of the first and second pure compounds, it is defined, on the one hand, a first spectral box made up of a first spectral zone delimited by values of at least these three determined spectral markers, and corresponding to the first pure component, and on the other hand, a second spectral box made up of a second spectral zone delimited by values of at least these three spectral markers, which were determined and that correspond to the second pure compound, at least for those three previously determined spectral markers and in a space corresponding to those spectral markers; - inside the conduit for conveyance of the product, at least one absorbance spectrum of this product circulating inside this conduit is recorded; - for at least one such absorption spectrum recorded for the product circulating inside the conduit, the value of at least three spectral markers corresponding to at least these three spectral markers previously determined for the first and second pure compounds is determined; - on a graph, the spectral boxes corresponding to at least one of the three spectral markers are represented, preferably at least the three spectral markers, for the first and second pure compounds; - on the same graph, the value of the spectral marker (s) determined and corresponding to at least one absorption spectrum recorded for the product circulating inside the conduit for conveyance is displayed; - the transition between the first compound and the second compound is detected for this product when the value of the spectral marker(s) determined and corresponding to this product moves away from the first spectral box and/or approaches the second spectral box, or even corresponds to this second spectral box.