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Insulating diamond has earlier been demonstrated as an efficient photoconductive material capable of switching up to several kilovolts with a subnanosecond response time. Here we report the results of a systematic study and comparison of the optoelectronic properties of specimens from the three major classes of insulating natural diamonds (types Ia, Ib, and IIa) and semiconducting diamond (type IIb) using picosecond 1.06, 0.53, 0.35, and 0.266μm pulses. The studies included measurements of the sensitivity at the different wavelengths, linearity as a function of light intensity up to 350 MW/cm2, linearity as a function of bias voltage up to approximately 2 kV and determination of the time response of the diamonds. The results allow us to conclude that at all wavelengths photoconductivity occurs through the presence of mid-gap defects or impurities and that multiphoton effects are negligible. The photoconductivity is observed to be entirely due to bulk mechanisms and role of electrical contacts or space-charge effects is found to be negligible except in the case of the IIa diamonds. The photoconductivity decay time is found to be essentially constant at a value of approximately 150 ps for all the insulating diamonds; this contrasts with the 50-300 ps range observed for the semiconducting stones.