The binary zirconium-tin system was reinvestigated. The A15 phase appears to be a line phase with a Zr[subscript]4Sn composition. This phase is extremely unstable with oxygen contamination. The Zr[subscript]5Sn[subscript]3 (Mn[subscript]5Si[subscript]3-type) and Zr[subscript]5Sn[subscript]4 (Ti[subscript]5Ga[subscript]4-type) compounds are line phases below 1000°C, the latter being a self-interstitial phase of the former. ZrSn[subscript]2 is the tin-richest phase. There is an one-phase region between these phases with partial self-interstitials at high temperatures. The zirconium-lead system behaves similarly: there are an A15 phase with a Zr[subscript]~5.8Pb composition, Zr[subscript]5Pb[subscript]3 (Mn[subscript]5Si[subscript]3-type) and Zr[subscript]5Pb[subscript]4 (Ti[subscript]5Ga[subscript]4-type) compounds, and a high temperature solid solution between Zr[subscript]5Pb[subscript]>3.5 and Zr[subscript]5Pb[subscript]4 from below 1000°C; however, the ZrSn[subscript]2 analogue is not formed;The Mn[subscript]5Si[subscript]3-type phases in these systems can accommodate third elements interstitially to form stoichiometric compounds Zr[subscript]5Sn[subscript]3Z (Z = B, C, N, O, Al, Si, P, S, Cu, Zn, Ga, Ge, As and Se) and Zr[subscript]5Pb[subscript]3Z (Z = Al, Si, P, S, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Ag, Cd, In, Sn, Sb and Te) as well as their self-interstitial derivatives. The stoichiometric nature of most of the interstitial phases was confirmed by structure analyses of single crystals for Z = C, O, and Ge in Zr[subscript]5Sn[subscript]3Z and Z=Zn and Pb in Zr[subscript]5Pb[subscript]3Z and of powder for Zr[subscript]5Sn[subscript]3Ga as well as by synthetic considerations. The crystal structures show strong zirconium-interstitial interactions. A substoichiometric interstitial phase also exists for Zr[subscript]5Sn[subscript]3S[subscript] x (0.5 ≤ x ≤ 1.0). Vapor phase transport reactions for Z = Al, Zn, and Pb in Zr[subscript]5Pb[subscript]3Z were successful with the transporting agent ZrCl[subscript]4;The systems Zr-Sn-T, T = Fe, Co and Ni, did not produce stoichiometric interstitial phases Zr[subscript]5Sn[subscript]3T. Instead, the interstitial phases for these elements are formed only with excess tin that partially occupies the interstitial site together with a T element. Reducing the amount of tin in these systems yields two new phases; Zr[subscript]5Sn[subscript] 2 + xFe[subscript] 1 - x (0 ≤ x ≤ 0.28) (W[subscript]5Si[subscript]3-type) and Zr[subscript]6Sn[subscript]2Fe (Zr[subscript]6Al[subscript]2Co-type) as characterized by X-ray single crystal analyses. A cobalt analogue for the latter was also synthesized. ftn*DOE Report IS-T-1505. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.