Abstract
Six transition metal dioxides, 
, 
, 
, 
, 
, and 
, have been examined as electrodes in 
solution. The oxides 
, 
,
, and 
have broad current‐potential profiles, indicating the formation of a surface layer which can exist over a range of compositions. Steady‐state measurements of 
reduction showed catalytic activities which were low compared to common 
catalysts such as Pt, but of the same order as other oxide catalysts. Activities were lowest for 
and 
, which form resistive surface layers of a higher oxide. The only material studied which was sufficiently stable to allow measurement of both 
evolution and 
reduction was 
. The current‐potential profiles of 
and 
are characterized by distinct changes of oxidation state. These two oxides were not sufficiently stable to allow the measurement of 
reduction.