The concept of a chemically acceptable model is developed, Chemically acceptable models are causal and maintain the nonnegativity of concentrations. An extension of the law of mass action allowing delayed effects is described and shown to lead to chemically acceptable models. Delayed variable enzyme catalysis and Oregonator models are studied and shown to be equivalent to their classical mass-action counterparts from a number of perspectives, This equivalence is shown to be a consequence of an exact correspondence between certain ordinary and distributed-delay differential equations. For sufficiently small delays, this correspondence extends to discrete-delay systems. However, reversible chemical reaction networks cannot be adequately modeled with discrete-delay equations.