The advent of cloud-based services stimulates dynamic interaction/composition of various enterprise applications. In this context, Quality of Service (QoS) monitoring is key, since assessing the actual quality of what service users are paying for has become a mission-critical business practice requirement, and it will be even more so in the future. QoS-MONaaS (QoS MONitoring as a Service) is a portable architecture that implements a dependable (i.e. unbiased, reliable, and timely) facility for monitoring the QoS being delivered at the Business Process level on top of a generic cloud platform. The architecture is portable in the sense that it can be ported from one platform to another with minimal modifications. The facility is made available to all applications running on top of a cloud platform according to the "as a Service" paradigm. This paper describes QoS-MONaaS conceptual architecture, and its implementation on top of the SRT-15 cloud platform, i.e. the cloud platform being developed within the context of the SRT-15 project (http://www.srt-15.eu/).