Canter and Fritzon ( Legal and Criminological Psychology , 3 , 73-96, 1998) proposed a new model of differentiating arsonists based on the action system model. This is a replication of their study utilising interviews with 65 inmates from one prison, HMP Grendon. A smallest space analysis (SSA) supported an action system model of four distinct modes, adaptive, expressive, integrative and conservative, reflecting Canter and Fritzon's four distinct themes of arson (Canter and Fritzon, 1998 ). Two themes relate to expressive acts-(a) those arsons targeted at a person and (b) those at an object. The remaining two relate to instrumental acts-(c) those targeted at a person and (d) those at an object. Using four scales, which related to the four themes, 55 (85%) of the acts could be assigned to either one or two adjoining themes. The study gives further support to the utility of the action system model and the model of firesetting behaviour proposed by Canter and Fritzon, when using a different method of data collection with a sample of serious offenders.