This paper presents QLSE (QoS-constrained List Scheduling hEuristics), a Quality of Service-based launch time scheduling algorithm for wide area Grids. QLSE considers applications described by a Task Interaction Graph (TIG) whose nodes and edges are labeled according to the Quality of Service requirements of the application. The high values obtained in the performance evaluation for both the tasks communication and computation throughput demonstrates the applicability of the proposed approach.