A technique has been developed for determining unambiguously the effective barrier height &phgr; and the average effective thicknessSof the insulating layer in a metal‐insulator‐metal tunnel junction. The quantityS&phgr;3/2is measured from the slope of the Fowler‐Nordheim plot. The experimental data are then compared with a family of calculated current‐voltage curves by using various pairs ofSand &phgr; withS&phgr;3/2fixed at the measured value, in order to determine the pair which gives the best fit. The method has been tested on the best understood tunnel barrier, those grown thermally on an Al film, and is found to give good agreement with independent measurements. From a macroscopic viewpoint, the tunnel barrier seems adequately described by a trapezoidal barrier model of nearly uniform height but with thickness variations on a microscopic scale over the junction area. The use of the method is illustrated on barriers grown thermally on Cr, thin‐film V, and bulk V. We present a series of graphs at varyingSand &phgr; for six different values ofS&phgr;3/2to facilitate the determination of approximate barrier parameters with lengthy computer calculations.