Paracetamol trihydrate is a highly unstable form which undergoes a rapid phase transformation to its stable anhydrous polymorph I. In situ Raman surface mapping on a trihydrate sample provides evidence for the existence of a transient phase on the route to its stable polymorph I. The transient phase showed significant spectral differences from those of known polymorphs, the trihydrate and amorphous forms, and is tentatively assigned to the monohydrate form on the basis of the observed crystallite morphology.