A 10‐&mgr;m detection system based on up‐conversion in proustite has been developed for the measurement of subnanosecond pulses of CO2laser radiation. This has involved the development of a ’’stretched‐pulse’’ Q‐switched Nd:YAG oscillator which can be synchronized with the output of a transverse discharge CO2laser. The 1.06‐&mgr;m radiation generated by this oscillator and the incoming 10‐&mgr;m radiation pass collinearly through a 0.6‐cm‐long proustite crystal which is oriented for type II phase matching. An investigation of the bandwidth of the up‐conversion process has shown that it should be possible to resolve 10‐&mgr;m pulses with durations of ∼5 psec. Using an image converter streak camera to record the 0.96‐&mgr;m sum frequency, the overall performance of the system has been investigated with the aid of a mode‐locked CO2laser operating at a pressure of 12 atm. The results have demonstrated that mode‐locked CO2pulses as short as ∼75 psec can be generated and that the temporal resolution of the complete detection system corresponds to that of the streak camera.