The diffusion of Pb and Hg was measured in Pb: Hg alloys of up to 8-at.% mercury between 200 and 300 \ifmmode^\circ\else\textdegree\fi{}C by the serial-sectioning technique. The diffusion of mercury in pure lead is well described by ${D}_{\mathrm{Hg}}={D}_{0}{e}^{(\ensuremath{-}\frac{{H}_{\mathrm{Hg}}}{\mathrm{RT}})}$, where ${D}_{0}=1.05\ifmmode\pm\else\textpm\fi{}0.24$ ${\mathrm{cm}}^{2}$/sec and ${H}_{\mathrm{Hg}}=22.7\ifmmode\pm\else\textpm\fi{}0.2$ kcal/mole, and is 15-20 times faster than the lead self-diffusion. The diffusivities of both lead and mercury are enhanced by increasing the mercury content of the alloys. This is incompatible with an interstitial mechanism. The enhancement coefficients, measured at four temperatures, are very small and lie close to the minimum allowed by a vacancy mechanism. The predictions of both vacancy and interstitial-vacancy pair models are reviewed and compared to the present data. This shows that neither model in its present form gives a fully satisfactory description of this system's diffusion behavior and so a possible extension of the interstitial-vacancy pair model is suggested.