The dimensionless ratio sigma(G), expressing quantitatively the effects of heterointeractions between unlike units on the conformational properties of copolymers, has been determined experimentally by intrinsic viscosity analysis for the A(2)B and A(3)B miktoarm stars in various solvent conditions. In the case of symmetric samples (where the lengths of A and B arms are the same) in a common good solvent, the values of the required Flory parameter Phi for homopolymer stars with three and four arms were used. For the other cases and solvent. conditions, Phi was determined by extended Monte Carlo calculations which provide a lower bound of this quantity. The experimental values of sigma(G) are compared with the results obtained by Monte Carlo simulations and recent renormalization group predictions.