Constraint automata have been used as an operational model for Reo which offers a channel-based framework to compose complex component connectors. In this paper, we introduce a variant of constraint automata with discrete probabilities and nondeterminism, called probabilistic constraint automata. These can serve for compositional reasoning about connector components, modelled by Reo circuits with unreliable channels, e.g., that might lose or corrupt messages, or channels with random output values that, e.g., can be helpful to model randomized coordination principles.