Two methyl derivatives, five-membered methyl 2-{2-[2-(9,10-dihydroacridin-9-ylidene)-1-methylhydrazinyl]-4-oxo-4,5-dihydro-1,3-thiazol-5-ylidene}acetate, C(20)H(16)N(4)O(3)S, (I), and six-membered 2-[2-(9,10-dihydroacridin-9-ylidene)-1-methylhydrazinyl]-4H-1,3-thiazin-4-one, C(18)H(14)N(4)OS, (II), were prepared by the reaction of the N-methyl derivative of 4-(9,10-dihydroacridin-9-ylidene)thiosemicarbazide, C(14)H(12)N(4)S, (III), with dimethyl acetylenedicarboxylate and methyl propiolate, respectively. The crystal structures of (I), (II) and (III) are molecular and can be considered in two parts: (i) the nearly planar acridine moiety and (ii) the singular heterocyclic ring portion [thiazolidine for (I) and thiazine for (II)] including the linking amine and imine N atoms and the methyl C atom, or the full side chain in the case of (III). The structures of (I) and (II) are stabilized by N-H...O hydrogen bonds and different pi-pi interactions between acridine moieties and thiazolidine and thiazine rings, respectively.