The solution-mediated polymorphic transformation of FV sulphathiazole was investigated in ethanol at 10 degrees C. Powder X-ray diffraction (PXRD) identified the transformation product as a mixture of FII and Fly sulphathiazole. This mixture remained stable in solution at 10 degrees C with agitation for 24 h. In situ temperature controlled optical microscopy enabled visualization of the transformation in real time, allowing elucidation of the transformation mechanism. A modest amount of dissolution of FV sulphathiazole preceded the nucleation of FII and FIV crystals, which were detected simultaneously. Thereafter, these polymorphs grew rapidly. Experimental evidence is presented for a mechanism, whereby nucleation of FII and FIV was initiated on the roughened surface of the dissolving FV crystals, without any indication for a spatially extended structural relationship between the host form V and the nucleating phases. Subsequent growth of FII and FIV crystals consumed the remaining FV crystals completing the dissolution-recrystallization mechanism.