Hazard or ‘risk’ quotients are commonly used to characterize the risk to ecological receptors at Superfund sites. A risk quotient is the ratio of an estimated exposure level (concentration or dose) to an effects threshold level (concentration or dose). Ecological hazard quotients can be formulated and interpreted in probabilistic terms. The numerator of the ratio is the exposure corresponding to the maximum allowable risk to the receptor at the site. Given a distribution of exposures at a site, the specified exposure level defines the maximum acceptable probability that a receptor experiences unacceptable harm, i.e., maximum acceptable risk. The exposure level at Superfund sites is frequently specified by regulators to be that corresponding to the upper 95th confidence limit on the mean concentration. The denominator of the ratio is the exposure corresponding to the dose causing the maximum allowable adverse effect on the receptor. Given a distribution of exposures at a site, the effects threshold defines the actual probability that the receptor is exposed to levels greater than the threshold, i.e., actual risk. The effects threshold at Superfund sites is frequently required by regulators to be that corresponding to the no effect level. This formulation of ecological hazard quotients clarifies the meaning of the quotient and highlights the importance of the exposure level and effects thresholds specified by regulators. This paper also discusses the relationship between this probabilistic interpretation of risk quotients and probabilistic exposure modeling, e.g., Monte Carlo simulation.