Information and service quality (ISQ) is a critical factor in the success of commercial web sites and is a well-researched areain Information Systems (IS) and Marketing. Our study develops an instrument that measures web site success by evaluatingISQ performance. We deploy a multi-dimensional conception of ISQ from IS literature that we use for measuring dimensionrelevance and performance perception according to Marketing research. Both are components of a linear compensatoryrelevance-weighting model. We extend this model by integrating a maximal performance norm and a normalizationprocedure for calculating an overall ISQ performance score, establishing a maximal perceived-performance gap. In addition,different web site usage contexts are linked to performance perceptions. The applicability of the approach is demonstrated inan empirical study examining the ISQ performance of a commercial web site (n = 226). Our instrument adds exploratorypower to existing measurement approaches and supports customer-centric web site design.