Poly(vinyl butyral) (PVB)was found to precipitate from a clear plasticizer solution as it was cooled and this unique temperature (T-P) (UCST) was noted. Poly(vinyl chloride) (PVC) did not precipitate and in this case the temperature at,which it went into solution (T-S) was noted. A quantity, referred to previously as the miscibility parameter (hip), is an excellent relative quantitative indicator of the miscibility of poly(vinyl butyral) and poly(vinyl chloride) with plasticizers as demonstrated from a correlation of hip with T-P and T-S. Each polymer-plasticizer system is unique. A plot of MP vs. T-P or T-S for several polymer-plasticizer systems allows additional plasticizers to he evaluated for their plasticizing ability from calculated MP’s without resorting to additional experimental measurements. The absolute level of hip acceptability is dictated by the application. For PVB, a copolymer which precipitates from a plasticizer/solvent, the interaction parameter chi(chi) is chi critical (chi(cr)) at the T-P of PVB in plasticizer. Since T-P approximates the Theta temperature in the limit of molecular weight, the temperature of precipitation or Theta temperature, the solvent volume V-S, and the chi(cr), define the theoretical MP(cr) as given by (chi(cr)-0.35)RT/V-S. All MP(cr)’s were approximate to 0.3 which represents the critical MP above which phase separation occurs. The low molecular weight plasticizers were estimated to drastically increase MP(cr) to about 1 for acceptable application compatibility and this value compares favorably with (PVB) acceptable for a wide range of plasticizer-PVB combinations based on a plasticizer DP of about 20.