Method for analysing a gas, with a view to estimating an amount ( c k) of a plurality of different gas species (G k), called the gas species of interest, potentially present in the analysed gas, each gas species being able to absorb light radiation in an absorption spectral band (Δ k), the method comprising the following steps: a) placing the analysed gas between a light source and a measuring photodetector, the light source being able to emit illumination radiation that propagates through the gas, the measuring photodetector being able to detect radiation transmitted by the gas and to deliver a detection signal representative of an intensity ( I j) of the detected radiation; b) illuminating the analysed gas using the light source, the illumination radiation occupying a spectral band, called the illumination spectral band (Δ 12), covering the absorption spectral band (Δ k) of a plurality of gas species of interest, one spectrum ( S j), called the illumination spectrum, of the illumination radiation corresponding to each illumination; c) during the illumination, detecting, by means of the measuring photodetector, an intensity ( I j) of radiation transmitted by the analysed gas, in at least one detection spectral band (Δ 20); d) reiterating steps b) and c), while modifying the illumination spectrum ( S j) of the illumination radiation; e) determining an amount ( c k) of each of the gas species of interest depending on the intensity ( I j) of the radiation detected during each illumination, each intensity ( I j) being associated with one illumination spectrum ( S j); the method being characterized in that step e) comprises: - during each illumination, estimating an intensity ( Î 0, j), called the unattenuated intensity, that would be measured by the measuring photodetector in the absence of gas; - during each illumination, estimating the intensity detected in step c), using a parametric response function f such that Î j = Î 0,j f θj,αj,k,nj,k ( c 1... c K), where ∘ Î 0,j is an estimate of the unattenuated intensity during said illumination; ∘ θ j is a parameter dependent on the illumination spectrum; ∘ α j,k and n j,k are parameters dependent on the illumination spectrum and on each gas species of interest, ∘ the parameters associated with two distinct illumination spectra ( S j, S j -1) being different and defined beforehand; - obtaining, for each illumination, a comparison between the intensity ( I j) detected by the photodetector, in step c), and the estimate ( I j) of said intensity; - determining the amounts of the gas species of interest by minimizing an arithmetic combination of each comparison. a) placing the analysed gas between a light source and a measuring photodetector, the light source being able to emit illumination radiation that propagates through the gas, the measuring photodetector being able to detect radiation transmitted by the gas and to deliver a detection signal representative of an intensity ( I j) of the detected radiation; b) illuminating the analysed gas using the light source, the illumination radiation occupying a spectral band, called the illumination spectral band (Δ 12), covering the absorption spectral band (Δ k) of a plurality of gas species of interest, one spectrum ( S j), called the illumination spectrum, of the illumination radiation corresponding to each illumination; c) during the illumination, detecting, by means of the measuring photodetector, an intensity ( I j) of radiation transmitted by the analysed gas, in at least one detection spectral band (Δ 20); d) reiterating steps b) and c), while modifying the illumination spectrum ( S j) of the illumination radiation; e) determining an amount ( c k) of each of the gas species of interest depending on the intensity ( I j) of the radiation detected during each illumination, each intensity ( I j) being associated with one illumination spectrum ( S j); - during each illumination, estimating an intensity ( Î 0, j), called the unattenuated intensity, that would be measured by the measuring photodetector in the absence of gas; - during each illumination, estimating the intensity detected in step c), using a parametric response function f such that Î j = Î 0,j f θj,αj,k,nj,k ( c 1... c K), where ∘ Î 0,j is an estimate of the unattenuated intensity during said illumination; ∘ θ j is a parameter dependent on the illumination spectrum; ∘ α j,k and n j,k are parameters dependent on the illumination spectrum and on each gas species of interest, ∘ the parameters associated with two distinct illumination spectra ( S j, S j -1) being different and defined beforehand; ∘ Î 0,j is an estimate of the unattenuated intensity during said illumination; ∘ θ j is a parameter dependent on the illumination spectrum; ∘ α j,k and n j,k are parameters dependent on the illumination spectrum and on each gas species of interest, ∘ the parameters associated with two distinct illumination spectra ( S j, S j -1) being different and defined beforehand; - obtaining, for each illumination, a comparison between the intensity ( I j) detected by the photodetector, in step c), and the estimate ( I j) of said intensity; - determining the amounts of the gas species of interest by minimizing an arithmetic combination of each comparison.