The cubic lattice parameters for the solid solution system K2IrCl6- K2PtCl6 vary approximately linearly with composition from a0 = 9.765�0.001 Ǻ for K2,IrCl6, to a0 = 9.748�0.001 Ǻ for K2PtCl6. The 35Cl n.q.r, frequencies show shifts from the frequencies observed in the pure compounds, these shifts being approximately linear with composition at each of the temperatures 77�K, 201�K, and 298�K. The shifts in frequency on going from 0-100% Pt are, for the chlorine atoms bonded to the iridium, +93 kHz, and for the chlorine atoms bonded to the platinum, +84 kHz. ��� The infrared stretching frequencies of the MCl62- groups also show shifts which are approximately linear with composition. The extrapolated shifts in going from 0-100% Pt are +11 cm-1 and +4.5 cm-1 for the Pt-Cl and Ir-Cl stretching frequencies, respectively. The n.q.r. and infrared results are best accounted for by small changes in the M-Cl bond lengths within the MCl62- group, with concurrent changes in a0, and possibly also in the positional parameter u. Re-examination of the variation of magnetic susceptibility with composition in the K2IrCl6-K2PtCl6 system is also reported, and reasons proposed for the form of this variation. ��� 35Cl n.q.r. frequencies are reported for Cs2IrCl6.