This paper describes a system for making accurate and reliable ac measurements of capacitance as a function of frequency, bias voltage, and temperature. The experimental apparatus consists of the basic capacitance measuring circuit and an information gathering device. The basic circuit includes a constant ac voltage source in series with the cell, a dc source for biasing the cell, an ac current amplifier in series with the cell for monitoring the current, and a lock‐in amplifier to measure the ac current. The capacitance device is interfaced with a microcomputer to facilitate handling large quantities of data in a short time. The computer is also used to increment the bias voltage for C–V measurements. The system has the capability of measuring capacitance to an accuracy of about 5 digits. We have used this system to study deep impurity levels in thin‐film, polycrystalline CdS/CuxS solar cells. The deep traps appear as anomalies in the otherwise linear l/C2 versus V plots. These levels play an important role in determining cell characteristics and may also influence the cell’s efficiency. The advantages of the capacitance–conductance system described in this paper are: (1) simple construction; (2) accurate calibration procedure; (3) ability to take data with 4 1/2 stable digits; (4) the admittance can be measured as a function of bias voltage, frequency, and temperature; (5) automation, i.e., computer controlled, and (6) the ability to accurately provide information on unstable samples.