Dehydration and electrical conductivity (&sgr;) experiments were performed on diopsides from India (In), Sri Lanka (Sri) and Russia (Ru) to study the effect of hydrogen (H) on &sgr;. Dehydration showed that hydrogen concentrations (CH) of In- and Sri-diopsides remained constant (~50 and ~900 H/106 Si), and CH in Ru-diopside decreased continuously from 1000 to 100 H/106 Si within 72 h. &sgr;'s were measured at T=1193--1473 K, P=0.1 MPa and fO2=10-9 MPa. Different changes in &sgr; with time were observed: the &sgr; of Ru-diopside was reduced by one order of magnitude within 72 h as crystals dehydrated; &sgr;s of In- and Sri-diopsides remained time-independent. Sri-diopside shows the highest &sgr; corresponding to its highest CH among the three crystals. Our results suggest an important effect of H on the &sgr; of diopsides. We interpret this as a result from the fast diffusion of H incorporated into diopside. Also the concentration and mobility of intrinsic charged defects may increase due to the presence of H. ¿ 1999 American Geophysical Union