An adaptive on/off delay-compensation technique is proposed to improve the performance of CMOS active rectifiers for wireless power transfer (WPT) systems. The effects of the on/off delays on the performance of the active rectifiers with either a parallel-resonant or a series-resonant circuit at the secondary coil are studied, which include power conversion efficiency (PCE), voltage conversion ratio (VCR) and output voltage ripple. By adding two feedback loops to the active diodes to generate the switched-offset currents for the comparators adaptively, both on- and off-delays are compensated for accurately against PVT variations and mismatches. As a design example, a fully integrated active rectifier for biomedical applications with a parallel-resonant secondary was fabricated in a standard 0.35 μm CMOS process. With an AC input that ranges from 1.8 to 3.6 V, the measured VCR is higher than 90% and the measured PCE is higher than 89.1% for a load resistor of 500 Ω. In particular, the PCE is increased by 9% compared to the active rectifier without using the proposed technique.