Objective. This aim of this study was to determine the effect of water contamination on the dimensional change over time and the corrosion properties of a gallium-based alloy (Galloy, SDI), compared to an amalgam (Tytin, Kerr). Methods. Normal and water-contaminated (10μL of dH2O during condensation) Galloy and Tytin samples were fabricated using a split metal mold and mechanical condenser. The normal samples were tested for dimensional change under dry and wet (immersed in dH2O) conditions. The water-contaminated samples were tested for dimensional change under only dry conditions. Linear measurements were made over a 7 day period using a stage-equipped dial micrometer. The corrosion potential (Ecorr), the corrosion current density (Icorr), the passive current density (Ip), the protective scale formation current density (Ips) and potential (Eps), and the breakdown potential (Ebd) were obtained from the polarization curves generated in oxygenated saline solution. Single factor ANOVA and Duncan's multiple range tests were used to determine differences between the various samples. Results. The water-contaminated Galloy samples contracted to a minimum of -3.42 ± 1.73 μm cm-1 in 5 h followed by expansion to a maximum of 22.56 ± 3.90 μm cm-1 at the end of 7 days. Normal Galloy and Tytin samples and water-contaminated Tytin samples exhibited rapid contraction during the first 24 h, followed by minimal change over the next 7 days. The Ecorr of normal and water-contaminated Galloy samples was more active than that of the normal and water-contaminated Tytin samples. The Icorr of the normal and water-contaminated Galloy samples was three orders of magnitude greater than the Icorr of normal and water-contaminated Tytin samples. The Eps for the water-contaminated Galloy and Tytin samples was more noble than for the normal Galloy and Tytin samples. The Ebd for the Tytin and water-contaminated Tytin samples was greater than the corresponding values for the Galloy and water-contaminated Galloy samples. Significance. The gallium-based alloy exhibited expansion if contaminated with water during the condensing and setting process. Post-setting exposure to water did not result in expansion of the gallium-based alloy. The alloy also exhibited a greater susceptibility to corrosion than the amalgam. Due to the possibility of delayed expansion, this material should be used cautiously, particularly in applications involving weakened tooth structure. © 1998 Academy of Dental Materials. Published by Elsevier Science Ltd.