A LEED study has been carried out on mixed lead and thallium overlayers. The observed patterns are consistent with earlier chain models of the pure materials and indicate that the mixed systems contain areas of interpenetrating chains with interaction between chains of one material despite intervening chains of the other. For the samples with a thallium content exceeding 50%, there is evidence for the formation of chains in which the Peierls distortion observed in pure thallium overlayers is suppressed.