cis-1,4-Dihydro-1,4-dimethyl-1,4-diarsinine-2,3,5,6-tetracarboxylic acid dianhydride was quantitatively formed from cis-1,4-dihydro-1,4-dimethyl-2,3,5,6-tetrakis(t-butoxycarbonyl)-1,4-diarsinine treated with formic acid. X-ray crystallography suggests that the six-membered ring in cis-1,4-dihydro-1,4-dimethyl-1,4-diarsinine-2,3,5,6-tetracarboxylic acid dianhydride was in a boat conformation and was nearly flat, and the As–C=C bond angle of cis-1,4-dihydro-1,4-dimethyl-1,4-diarsinine-2,3,5,6-tetracarboxylic acid dianhydride extended to 133°. Polymerization of cis-1,4-dihydro-1,4-dimethyl-1,4-diarsinine-2,3,5,6-tetracarboxylic acid dianhydride with 1,3′-bis(4-aminophenoxy)benzene at 80 °C in DMAc for 12 h yielded a soluble polyimide, of which number-average and weight-average molecular weights were estimated to be 23 400 and 60 300, respectively. The polyimide was obtained even when polymerization was carried out at 60 °C for 24 h.