Huang et al. (2001) proposed a deadlock prevention policy based on Petri net siphons for flexible manufacturing systems (FMS). The current paper presents an analysis to show that in terms of control, the proposed policy appears to be more permissive than previous siphon-based approaches. Specifically, two siphon-based methods are compared with this approach. One is Ezpeleta et al.'s (1995) work for S/sup 3/PR nets and the other is Huang et al.'s (2001) work called a two-stage policy due to its two stages of preventing deadlocks. All these three rely on the addition of control places to the original net model to prevent siphons from being unmarked. The differences lie in the exact locations and sequences of adding the control places. Our analysis starts with the development of several theoretical results followed an experimental study. Through reachability analysis, all of the experiments indicate that the proposed policy appears to be more permissive than the other two. In addition, the two-stage policy appears to be more permissive than Ezpeleta et al.'s approach.