The reproducibility of VIZ radiosonde data has been investigated using three ascents with two sondes on the same balloon. Provided the baseline check is carried out carefully and under favorable conditions, the reproducibility of the VIZ sonde is quite satisfactory with the deviations well within the accuracies specified by the manufacturer. While no serious deviations in pressure and temperature were found, there were significant discrepancies between the humidity data from the two sondes. Further investigations revealed that the accuracy of humidity data strongly depends on the calibration point chosen for the baseline check. It was verified that the optimal calibration point is between 30 and 40% RH as recommended by the National Weather Service. Also, at below freezing temperatures and high humidities that the analytical transfer function for the old VIZ humidity sensor (produced before June 1980) is inadequate, giving too low humidities values. Laboratory experiments and comparison with other radiosondes showed that temperature of VIZ sondes should be corrected for radiation effects; a correction table has been compiled using data from day/night comparisons of other authors and from the laboratory experiments.