Optical Faraday rotation of rare‐earth (III) borate glasses has been investigated at room temperature. The rotation is described in terms of an effective transition wavelength &lgr;twhich is an indicator of the spectral region of the 4fn‐4fn−15dabsorptions mainly responsible for the rotation. An experimental investigation of the magnitude of rotation as a function of rare‐earth (III) ion reveals the importance of the transition wavelengths and the electric dipole matrix elements as parameters affecting the rotation. The borate matrix is found to be more effective than many other environments in the realization of higher Verdet constants.