Two types of (acridin-9-yl)thiosemicarbazides with the acridine moiety in the thiourea part (Acr-NHCS, 10a, 10b) and hydrazine part (Acr-NHNHCS, 12a-12c) were prepared to investigate their reactions with dimethyl acetylenedicarboxylate. Five-membered thiazolidinone derivatives 15a, 15b, 19a-19c were formed; some aspects of corresponding reaction mechanisms are discussed. 1D and 2D 1H and 13C NMR spectroscopy and DFT quantum chemical calculations were used to elucidate the structure of the compounds.