A general method for calibrating Sun photometers that relaxes the constraints on atmospheric conditions is described. Instead of requiring constant extinction conditions the method requires only that the relative aerosol size distribution remains constant during observations over a range of air masses during a morning or afternoon. Provided that the relative aerosol extinction component [m(a)(t)δ(t, λ(0))] can be obtained at wavelength λ(0), the calibration at wavelength channel λ can be calculated with simple least-squares techniques. A variant of the method in which a Sun photometer is used to provide [m(a)(t)δ(t, λ(0))] is detailed and is verified with both model atmospheres and Sun-photometer data for 1988-1991 from Cape Grim, Tasmania (41°S). The method produces calibration data having sample variances more than 5 times smaller than Langley method calibration results.