The theory of nonideal, multicomponent, isotopic mixtures is used to calculate the vapor pressures of pure Kr-80(l) and Kr-84(l) from data on isotopic mixtures. The correction to ideal solution behavior, Delta, is found to be much smaller than the statistical deviations in the experimental data on the isotopic mixtures. It amounts to about 0.0005 and 0.0007 mmHg for the absolute vapor pressures of the pure isotopes at 116 and 123 K, respectively. The vapor pressure difference between pure isotopes is calculated to be 0.55(72) mmHg at 116 K after correction for nonideality compared with 0.55(73) mmHg based on ideal solution behavior. The corresponding differences are 0.83(81) and 0.83(82) mmHg, respectively, at 123 K. The theoretically important quantity, ln(p(80)Kr(l)/p(84)Kr(l)), shows a decrease (almost irrespective of temperature) of about 0.01% if nonideality is taken into account. The pressure-temperature data for normal krypton given by Lee, Eshelman, and Bigeleisen [J. Chem. Phys. 56, 4585 (1972)], in the temperature range 123.93-129.89 K cannot be reconciled with their vapor pressure equation for the normal liquid. We conclude that the Delta-correction can be safely discarded in the case of the vapor pressure isotope effect (VPIE) studies involving isotopic mixtures of krypton. Moreover, one can infer from this study that, in the case of the rare gases family, the borderline between still measurable and totally negligible nonideal behavior lies between the VPIEs found in mixtures of argon and those in mixtures of krypton, respectively. We anticipate that the case of neon isotopes deserve investigation since the deviations from ideality are expected to be about 400 times greater than those here predicted for krypton.