Tetrahydrofuran (THF) may be polymerised cationically by suitable initiators via a ring-opening mechanism to give linear, high molecular weight products which normally have a broad molecular weight distribution. Factors affecting the molecular weight distribution are discussed, and pertinent kinetic experiments are described, which allow the formulation of a procedure of synthesising monodisperse polymers of THF. It was found that the major causes of broadening were propagation-depropagation equilibria, chain transfer to polymer oxygen and slow initiation. When p-chlorophenyldiazonium hexafluorophosphate was used as initiator, a high initiation temperature followed by rapid quenching to a low propagation temperature, together with low conversion gave the required monodispersity. Kinetics of propagation were investigated over a wide temperature range, -10 °C to 80 °C, and the variation of kp with temperature found to follow an Arrhenius expression with an activation energy of 51 kJ.mol-1. When using triethyloxonium hexafluorophosphate as initiator, no separate initiation stage was necessary, the reaction proceeding at -5 °C to yield monodisperse polymers directly at low conversions.