In-flight measurements of the HCN conversion efficiency of three gold-tube NOy converters were made during the recent TOTE/VOTE and SUCCESS aircraft projects. While sampling ambient air and with the upstream addition of 500 ppmv H2O, two converters had HCN conversion efficiencies averaging about 5%. The H2O addition was a routine part of the NOy measurement procedure for these projects. A third converter showed much higher HCN conversion, about 30%. Conversion efficiencies determined while sampling synthetic air were significantly higher than those measured while sampling ambient air. The dependence of conversion efficiency on ambient H2O and O3 was minimal, at least with respect to the variability of the measured values over the course of the missions. ¿ 1998 American Geophysical Union