Dissolution of lamellar precipitates by cell boundary migration in a Pb-5.5 at. pct Sn alloy has been studied in the temperature range between 44° and 83°C by optical microscopy. Morphological observation shows that it is a cellular mode of transformation. The cell boundary, acting as a short circuit path of diffusion, recedes, thereby dissolving the precipitates. The orientation of the transformed matrix is found to follow the same relationship as that in cellular precipitation. Measurements of the rate of dissolution are in agreement with a kinetic analysis that assumes a boundary diffusion limited process. A critical temperature, defined as the temperature at which the receding boundary ceased moving, was found to exist slightly below the solvus temperature of the alloy. By balancing driving forces at both sides of the cell boundary at the critical temperature, we evaluated the upper limit of the interlamellar surface tension of Pb-Sn alloy as 232 erg per sq cm. This value is six to ten times smaller and is believed to be more realistic than those values calculated from the precipitation kinetics.