Cycle-harvesting is a significant part of the Grid computing landscape. However, creating commercial service contracts based on resources made available by cycle- harvesting is a significant challenge: the characteristics of the harvested resources are inherently stochastic; and secondly, in a commercial environment, purchasers can expect providers to optimize against the quality of service (QoS) definitions. The essential point for creating commerc ally valuable QoS definitions is to guarantee a set of statistical parameters for each contract instance. Here we describe an appropriate QoS definition, Hard Statistical QoS (HSQ), and show how this can be implemented using a hybrid stochastic-deterministic system. We analyze algorithm behavior analytically using a distribution-free approach versus the expected proportion of deterministic resources required for an HSQ specification. We conclude that commercial service contracts based on cycle-harvested resources are viable both from a conceptual point of view and quantitatively.