Comparative data are reported from a group of twin or dual radiosonde observations made with U.S. Weather Bureau and military 1680me./sec. radiosondes. To compare the instruments directly, differences in temperature, pressure, and relative humidity are studied at simultaneous time marks during the observations. Root-mean-square differences of pressure, temperature, and relative humidity for each observation are summarized in table form. The root-mean-square differences for all observations combined are 2.1 mb. and 0.51° C. Tables show temperature and height differences evaluated at constant pressure surfaces. These temperature differences are somewhat greater than those at simultaneous time marks, as would be expected with the permitted tolerances and the judgment required in placing levels. Moreover, temperature differences obtained by this method are modified by pressure differences. Both types of radiosondes give compatible measurements, at least to the levels reached. The radiosondes were obtained from field stock, were not changed or adjusted in the laboratory, and were flown in the same manner as for routine soundings except for the dual feature. Certain laboratory tests were conducted and factory calibration data were obtained from these radiosondes. These tests and data are reported but were not used in the evaluations of the observations.