Experimental results are described which serve to emphasize the importance of taking certain precautions when attempting to use crystal rectifiers in measurement work. Particular attention is drawn to the effect of the r.f. circuit impedance on the apparent rectification law and to that of temperature on the rectification sensitivity. The main conclusion drawn is that if crystal rectifiers are to be used in measurements in which their rectification law is in any way involved, considerable care must be taken to ensure correct working conditions. In this connection certain general recommendations are made.