Terpolymer composition estimation with an established equation has been found to give results that vary with the feed monomer ratio substitution pattern used. A new copolymer equation has been derived that overcomes this difficulty to give the same composition results regardless of the order of monomer substitution. The new equation also gives comparable or better agreement with experimentally determined copolymer compositions than obtained by use of the established Alfrey-Goldfinger terpolymer equation. In addition, this new terpolymer equation demonstrates a versatility not shown by the present terpolymer equations in that it can be readily adapted by inspection ro enable estimation of copolymer compositions for two- or for four-component polymer systems. It is also readily adaptable for copolymers with more than four components, and may also be valid for composition estimation of these because of the derivation method used, although it has not as yet been possible to test this possibility.