Eight new cis-methylplatinum(II) derivatives of the type DIARSPt(CH3)X (DIARS abbreviates o-phenylenebis(dimethylarsine), where X is an anionic substituent, and [DIARSPt(CH3)L]+PF6−, where L is a neutral ligand, have been prepared. Data derived from the 13C n.m.r. spectra of these complexes are discussed and compared with data derived from the 1H n.m.r. spectra of the complexes. Comparisons with the 13C n.m.r. parameters obtained from related series of trans-methylplatinum(II) derivatives reveal marked differences between the n.m.r. cis- and trans-influences. It appears that the n.m.r. cis-influence may in part be related to a ligand's "steric bulk".