Positron trapping at structural defects in lead, tin and some lead-tin alloys has been studied by measurements of the temperature dependence of the Doppler broadening of the annihilation photon line. Monovacancy formation enthalpies have been deduced from the data. In a Pb-8 at.% Sn alloy a sharp transitory peak in the peak height to area ratio or S parameter is observed some 20 degrees below the solvus temperature. An interpretation is offered in terms of enhanced positron trapping at grain boundary sites associated with the initial stages of tin precipitation.