Liquid-liquid coexistence curves of the system (tetrahydrofuran + water) and the pseudo-binary system (tetrahydrofuran + deuterated tetrahydrofuran + heavy water) were measured in the reduced temperature range 3 X 10(-5) < tau < 7 X 10(-2) from their lower critical points. The composition of the deuterated system was adjusted to get the critical temperature close to that of the normal binary mixtures. This ensures that the effect of the upper consolute temperatures is the same in both cases, thus facilitating data interpretation. The refractive index and the Lorentz-Lorenz function were chosen as composition variables for analyzing the order parameter. For tau < 10(-3) both mixtures exhibit pure Ising behavior. The leading critical amplitude increases in the deuterated mixture. The crossover behavior is found to be sensitive to changes in deuteration.