The stratigraphical limits on the age of the Freetown intrusion, Sierra Leone, are very wide, yet the intrusion has not previously been accurately dated by isotopic methods, despite a number of attempts. Rubidium-strontium dating of acid veins contemporaneous with the early stages of the prolonged cooling history of the intrusion provides an age of 193 ± 3 Ma. The veins consist of quartz and orthoclase with relict minerals, principally plagioclase, from thehost gabbro. Electron-microprobe analysis of the altered minerals of the veins, and the petrography of the vein and adjacent host gabbro clearly demonstrate that the veins were formed from a granitic fraction, differentiated in situ from the surrounding solid gabbro with the assistance of a hydrous fluid phase within the incipient vein. This assertion is supported by the identical, low value of the initial ⁸⁷Sr/⁸⁶Sr ratio (0·70389) obtained from both the acid and basic rocks, and the technique described here may be useful in dating other, similar, intrusions.