Although services play a crucial role in developed countries, service productivity management is not as much researched as productivity management in production systems. Especially complex, knowledge intensive service systems have specific characteristics that have to be considered, in order to comprehensively evaluate their performance. A novel productivity model considering quantitative as well as qualitative aspects from both customer and provider side lays the foundation for a simulation based prospective productivity assessment of complex services. The paper concludes by presenting a use case scenario from the chemical industry.