The performance of unstructured mesh applications presents a number of complexities and subtleties that do not arise for dense structured meshes. From a programming point of view, handling an unstructured mesh has an increased complexity to manage the necessary data structures and interactions between mesh-cells. From a performance point of view, there are added dif.culties in understanding both the processing time on a single processor and the scaling characteristics. In this work we present a performance model for the calculation of deterministic S_N transport on unstructured meshes. It builds upon earlier work that successfully modeled the same calculation on structured meshes. The model captures the key processing characteristics and is parametric using both the system performance data (latency, bandwidth, processing rate etc.) and application data (mesh size etc.) as input. The model is validated on two clusters (an HP AlphaServer and an Itanium- 2 system) showing high accuracy. Importantly it is also shown that a single formulation of the model can be used to predict the performance of two quite different implementations of the same calculation.