Flatband potential, V-fb, measurements of n-CdSe in alkaline ferrocyanide electrolytes substantiate trends observed for photoelectrochemical open circuit voltage, V-oc, with dissolved cyanide. Similar trends comprising a negative shift of V-fb and an increase in V-oc with increasing concentration of cyanide are also observed for an n-CdTe alkaline ferrocyanide photoelectrochemical cell, PEC. Modification of an alkaline ferro/ferricyanide electrolyte with KCN, in an n-CdSe or n-CdTe PEC, stabilizes the photocurrent, and increases the V-oc by several hundred millivolts. In the approach to the high cyanide concentration limit, V-fb for both n-CdSe and n-CdTe in alkaline ferrocyanide approaches a single limiting value of -1.2 V vs ferro/ferricyanide.