The Mössbauer effect of 57Fe in aluminum was measured over the temperature range 20-642°C, using a source specimen of aluminum with 57Co in solid solution. The line-broadening values were converted to diffusion constants, which can be expressed by the temperature dependence D=0.12e-(1.4 eV) / kT cm2/sec. This result differs from recent diffusion-constant determinations obtained by the tracer-sectioning method, which we feel might be in error because of solution trapping. From theoretical considerations we suggest that a new expression for the correlation factor for the diffusional line broadening might be more accurate than the one used heretofore. Earlier determinations of the second-order Doppler shift were extended almost to the melting point of aluminum. The shift depends almost linearly on temperature, with the slope given by the Dulong—Petit rule for the heat capacity. From the temperature dependence of the Lamb—Mössbauer factor an effective Debye temperature of (210 ± 15)°K was found.