Method of detecting the presence and/or determining the amount of a non-prevailing polymorphic form of a polymorphic compound in the presence of one or more prevailing polymorphic forms, comprising the following steps: providing a sample comprising said polymorphic compound in powder form or in a shaped solid form or in a form in which the solid polymorphic compound is suspended, dispersed or mixed with a liquid; providing reference information from XRD analysis on the position of marker peaks of said non-prevailing polymorphic form of a said compound in a standard intensity versus scattering angle plot in relation to peaks of said one or more prevailing polymorphic forms; providing a beam of a synchrotron radiation from a synchrotron source, said synchrotron radiation having a wavelength in the range from 0.5 to 3.0 Å. exposing said sample to said beam with a spot size at said sample from 100 μm 2 to 0.1 cm 2, said spot size being selected by using slits or pinholes; checking the extent of possible damage caused by the exposure of the sample to the synchrotron radiation; collecting the intensity and scattering direction of the diffracted radiation using a X-ray detector, while optionally rotating said sample in order to probe different orientations of the sample volume hit by the beam; moving one or more times said sample with respect to the said beam to expose different areas of said sample to said beam and balance possible inhomogeneities of the sample, and repeating step for different areas of the sample; Processing the intensity and scattered direction data of said radiation collected by said detector to generate at least one plot reporting the scattered X-ray intensity versus scattering angle; screening said plot to detect one or more marker peaks of said non-prevailing polymorphic form of said compound at scattering angles at which said one or more prevailing polymorphic forms do not have diffraction peaks; optionally processing the data corresponding to said one or more marker peaks to quantify said non-prevailing polymorphic form of said compound with respect to the prevailing form. providing a sample comprising said polymorphic compound in powder form or in a shaped solid form or in a form in which the solid polymorphic compound is suspended, dispersed or mixed with a liquid; providing reference information from XRD analysis on the position of marker peaks of said non-prevailing polymorphic form of a said compound in a standard intensity versus scattering angle plot in relation to peaks of said one or more prevailing polymorphic forms; providing a beam of a synchrotron radiation from a synchrotron source, said synchrotron radiation having a wavelength in the range from 0.5 to 3.0 Å. exposing said sample to said beam with a spot size at said sample from 100 μm 2 to 0.1 cm 2, said spot size being selected by using slits or pinholes; checking the extent of possible damage caused by the exposure of the sample to the synchrotron radiation; collecting the intensity and scattering direction of the diffracted radiation using a X-ray detector, while optionally rotating said sample in order to probe different orientations of the sample volume hit by the beam; moving one or more times said sample with respect to the said beam to expose different areas of said sample to said beam and balance possible inhomogeneities of the sample, and repeating step for different areas of the sample; Processing the intensity and scattered direction data of said radiation collected by said detector to generate at least one plot reporting the scattered X-ray intensity versus scattering angle; screening said plot to detect one or more marker peaks of said non-prevailing polymorphic form of said compound at scattering angles at which said one or more prevailing polymorphic forms do not have diffraction peaks; optionally processing the data corresponding to said one or more marker peaks to quantify said non-prevailing polymorphic form of said compound with respect to the prevailing form.