We look at 1-region membrane computing systems which only use rules of the form Ca → Cv, where C is a catalyst, a is a noncatalyst, and v is a (possibly null) string of noncatalysts. There are no rules of the form a → v. Thus, we can think of these systems as "purely" catalytic. We consider two types: (1) when the initial conÿguration contains only one catalyst, and (2) when the initial conÿguration contains multiple catalysts. We show that systems of the ÿrst type are equivalent to communication-free Petri nets, which are also equivalent to commutative context-free grammars. They deÿne precisely the semilinear sets. This partially answers an open question (in: WMC-CdeA'02, Computationally universal P systems without priorities: two catalysts are su cient, available at http://psystems. disco.unimib.it, 2003). Systems of the second type deÿne exactly the recursively enumerable sets of tuples (i.e., Turing machine computable). We also study an extended model where the rules are of the form q : (p; Ca → Cv) (where q and p are states), i.e., the application of the rules is guided by a ÿnite-state control. For this generalized model, type (1) as well as type (2) with some restriction correspond to vector addition systems. Finally, we brie y investigate the closure properties of catalytic systems.