An approximate expression for the 2-terminal r.f. impedance of a thin Gunn diode is derived. From this, one can deduce a parallel equivalent circuit for the diode. The results are compared with the 1-dimensional model of Engelmann and Quate, and it is shown that the finite transverse dimensions of the diode can considerably affect its shunt capacitance. This result is applied to rederive a known stability criterion for thin Gunn diodes, and to correct the l.s.a. conditions of Copeland if thin diodes are being used. © 1970, The Institution of Electrical Engineers. All rights reserved.