The transfer function of optical fibre is now more frequently being specified in terms of frequency response. While measuring the amplitude over frequencies up to several GHz is quite straightforward, determining the phase response presents difficulties. This paper examines the nature of the practical problems associated with measuring the differential phase across a long fiber and shows how they can be overcome. A measurement system is described which performs the required measurements easily and quickly with a minimum of samples. Some results are given for 900 m of fibre over the range 25 to 1250 MHz and using these the capability of the system is compared with a time domain measurement. The frequency domain technique appears to be the superior method.