Design considerations for the multiplexing of pH-sensitive ISFETs are studied and discussed. Experimental results with an ordinary multiplex circuit show transients of the order of several seconds if the output voltage of the applied amplifier is required within an accuracy of 0.5 mV (corresponding to 0.01 pH unit). These transients appear to result from thermal instability of the ISFETs during switching operations, and can be cancelled by guaranteeing a constant chip dissipation. This necessitates the design of a special multiplexer circuit for multiplexing separate ISFETs. The output voltage of the applied amplifier then appears to be stable within 1 millisecond with the required accuracy.