Reactions of 1-phenyl-3,5-prazolidinedione with N-bromosuccinimide as well as carbonyl compounds were carried out. 4-arylidene-1-phenyl-3,5-pyrazolidinediones were reacted with alkylating agents and/or acylating reagents to give the N-alkyl and/or N-acyl derivatives. The electronic absorption spectroscopy of various derivatives was investigated.