Breakthrough curves (BTCs) for reactive solutes commonly show earlier appearance, greater spreading, and more tailing than can be represented by the classical advection-dispersion equation. Various types of kinetic models have been developed as a means of representing this nonideal behavior; however, several inconsistencies between the predictions of these models and experimental results have been noted. This paper presents an alternative mechanism that can contribute to the nonideality of BTCs for reactive solutes. The proposed mechanism is based on pore scale variation in the retardation factor (R) resulting from the pore size distribution. The effect of the mechanism on reactive solute transport was investigated using a stochastically derived macrodispersivity equation. In the example presented, the calculated stochastic dispersivity, which accounts for the pore scale variation in R, was almost 1 order of magnitude greater than that determined for a nonreactive solute. The BTC predicted using the stochastically derived dispersivity showed much better agreement with a measured BTC for strontium than that predicted using the nonreactive dispersivity. The simulated curve with stochastically derived dispersivity exhibited the common characteristics of nonideality, including enhanced spreading, early breakthrough, and tailing. Although the analysis was based on a highly idealized model of a porous medium, it is concluded that pore size variation in R can contribute to nonideality in reactive BTCs, even in a homogeneously packed column under conditions of equilibrium solute partitioning. ¿ American Geophysical Union 1995