Simultaneous observations at 326.5 MHz of the intensity variations of seven pulsars have been conducted over a 8000-km baseline between Australia and India. The theoretical interpretation of the drifting diffraction pattern is developed in some detail and a practical method is determined for measuring the velocity component along the baseline in the presence of intrinsic pulsar variability and random evolution of the pattern. It is shown that, because of the intrinsic variations, the usual method of determining velocity from the maximum cross-correlation leads to a large overestimate of the velocity. For the majority of the pulsars observed, the intensity fluctuations at the two stations were significantly decorrelated for all time shifts. It is concluded that the most likely reason for the decorrelation is the presence of a scale size approximately equal to our receiver separation of 8000 km. Three of the seven pulsars provided correlation functions which could be processed by the proposed method to give the velocity of pattern drift along the baseline. These velocities range from 40 km s −1 to 121 km s −1 and, if pattern velocity is indicative of pulsar velocity, are not high enough to place these pulsars in a class of particularly high-velocity objects. The pulsar PSR 1642 – 03 provided clear diffraction patterns whose direction of drift reversed in an interval of about 1 hr. This behaviour is not consistent with a simple model of the interstellar electron irregularities but it may be possible to explain it with a multi-screen model.