Precise measurements of the pressure in solid solutions of [SUP4]He in [SUP3]He are performed with temperature cycling in the phase-separation range. It is found that as a result of such cycling the difference DP of the pressures between the minimum (∼100 mK) and maximum (∼200 mK) temperatures decreases by a factor of approximately 3 and then remains unchanged for a long time. The initial values of ΔP and the pressure P[SUP0] in a uniform solid solution [SUP4[He-[SUP3]He are restored only after the sample is heated to a temperature substantially above the phase-separation point. The data obtained are explained on the basis of the hypothesis that under these conditions clusters of pure [SUP4]He form around quasiequilibrium vacancies. A thermodynamic calculation of such clusters is performed. The computational results made it possible to give a quantitative explanation of the distinctive features observed. Comparing the calculations with experiment made it possible to determine the cluster concentration, 8×10[SUP-5], and the cluster radius, which is 3-4 interatomic distances at low temperatures. The characteristic equilibration time under these conditions is determined, and it is suggested that processes occurring at cluster boundaries determine its value. [ABSTRACT FROM AUTHOR]