Electrochemical properties of sol-gel nanocrystalline cadmium tin oxide electrodes (CTO) were investigated in 0.2 M potassium chloride buffered at pH 7.4 with phosphate. Films were found to be n-type degenerate semiconductors with charge carrier levels from 10(19) to 10(22) cm(-3) depending on the thermal aftertreatment. X-ray diffraction analysis was used to reveal the appearance of the cubic cadmium stannate (Cd2SnO4) phase at annealing temperatures above 600 degreesC, and to indicate the extent of this dominant phase above 750 degreesC. The flatband potential (E-FB,) of the film electrodes, as determined from capacitance measurements, was found to be around +0.25 V at pH 7.4. Electrochemical activity toward ten redox processes in the range -0.45 V < E < 0.45 V was investigated, and standard electron transfer rate constants were estimated from ac impedance measurements. The dominant factor in the charge-transfer rate on CTO electrodes is the bull, film charge carrier concentration. It was found that the charge-transfer rates were dependent on the separation of the redox carrier formal potential (E-o') from the CTO flatband potential. The slowest rates (approximate to 10(-5) cm s(-1)) were found for redox couples where E-o' approximate to E-FB. For charge transfer from redox couples where E-o' is away from E-FB, the rates can be several orders of magnitude greater and it is thought that the density of states in the conduction band is rate limiting.