The CgP(Se)X series (X = H, F, Cl, Br and I; CgP = 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl) is generated by the addition of elemental Se to the corresponding CgPX. The crystal structures of CgPX (X = H, F, Cl, Br and I) and CgP(Se)X (X = F, Cl, Br and I) are compared. The JP,Se values are correlated with the P–Se distance and the electronegativity of X. For both the CgP(Se)X and the CgPX series, the P–C bond lengths correlate with the P–X bond lengths and with the C–P–C bond angle. These trends can be explained in terms of the δ+ charge on the P atom, which has been calculated in the gas phase. Cone-angle measurements suggest that CgPCl, CgPBr and CgPI have essentially the same steric bulk.