In elementary reactions, constraints on product rotation may occur when the reactants have large reduced mass while that of the products is very low. In these circumstances the energy conservation on recoil orbital angular momentum may severely restrict access to product rotation states. This is illustrated for Ba + HI -> Bal + H demonstrating that the constraint is highly (product) rotational state specific and ultimately determines which product states may be populated. When this energy conservation condition is properly included, predicted peak j-values and distribution widths agree very well with experiment. (c) 2007 Elsevier B.V. All rights reserved.