The crystal structures of two cobaloximes, [CoMe(Hdmg)2(PMe3)](1) and [CoMe(Hdmg)2{P(C6H11)3}](2)(H2dmg = dimethylglyoxime), are reported and discussed. Compound (1) crystallizes in the space group P with cell parameters a= 15.830(8), b= 12.279(7), c= 12.257 Å, α= 94.85(7), β= 84.49(8), γ= 130.07(9)°, and Z= 4; (2) crystallizes in the space group P212121 with a= 18.50(1), b= 16.83(1), c= 9.943(8)Å, and Z= 4. Both structures have been solved by Patterson and Fourier methods and refined by block-diagonal anisotropic least-squares methods to final R values of 0.037 (1) and 0.038 (2), using 4 812(1) and 2 710(2) independent reflections. The Me–Co–P fragment of (1) is characterized by a C–Co–P angle of 178.9(2)°(mean) and Co–P and Co–C bond lengths of 2.293(1)(mean) and 2.015(3)Å(mean); the corresponding figures for (2) are 179.1(2)°, 2.463(1), and 2.016(5)Å respectively. The value of 2.463(1)Å is the largest so far reported for a CoIII–P bond length. The two nearly planar Hdmg units make a mean interplanar angle of 4° in (1) and are bent towards the axial methyl group. A similar but more pronounced bending is observed in (2). The trend in Co–P distances in these and other similar complexes [CoX(Hdmg)2(PR3)](X = Cl or Me; R = OMe, Me, Bun, Ph, or C6H11) is discussed and compared with 31P and 1H chemical shifts. Deformations observed in the geometry of these octahedral cobalt(III) complexes are interpreted in terms of ‘steric’ and ‘electronic’ influences. Finally, kinetic measurements of the conversion of [CoMe(Hdmg)2{P(C6H11)3}] to [CoMe(Hdmg)2(py)](py = pyridine) give no indication of a significant contribution from steric effects on the rate of the five- to six-co-ordinate step. However, the relatively high rate of displacement of the P(C6H11)3 ligand compared to other phosphines of smaller cone angle almost certainly arises from the steric interaction of the C6H11 rings with the Co(Hdmg)2 moiety.