A new method is proposed for measuring the widths, and possibly even the detailed shapes, of picosecond pulses in CW mode-locked lasers. The pulses are passed through an electro-optic modulator, which is biased for zero transmission with zero applied voltage, and which is driven at a harmonic of the pulse-repetition frequency. The variation in average light transmission through the modulator is monitored while the relative phase between the light pulses and the modulator drive is varied. The pulsewidth may be deduced from one such measurement made at a microwave light-modulation frequency sufficiently high that the pulsewidth is a finite fraction of a period at the modulation frequency. By making such measurements at a number of harmonics, the complete Fourier expansion (including both amplitudes and phases) of the picosecond pulse envelope can, at least in principle, be determined without ambiguity.