A process of forming the cis- or the trans-isomers of thiazine-indigo compounds comprising the step of reacting o-aminomercapto-carbocyclic or -heterocyclic compounds with a maleic acid or fumaric acid derivative in the presence of an aqueous system. The invention further relates to a process of converting cis-isomers of thiazine-indigo compounds into the trans-isomers being pigments. The invention also relates to the cis-isomers of thiazine-indigo compounds of formula I as defined in claim 14 which are new compounds.