The irreversible thermodynamics of a nonlinear diode are discussed by considering a circuit in which a biased diode exchanges noise power with a linear resistor at a different temperature. By applying the Onsager reciprocity relations to the two cross terms involving the rectification of thermal noise, and the generation of current noise by the diode, respectively, an equation is obtained relating the nonlinearity of the diode and a parameter which specifies its current-noise behavior. The implications of this equation are discussed in relation to the existence of a maximum value of the nonlinearity. The analysis predicts that a reverse current will flow through the diode when the resistor is at a lower temperature, and an experiment to confirm this is reported briefly. © 1968 The American Institute of Physics.