As approaches and tools for process and enterprise modelling are maturing, these techniques are in an increasing number of organizations being taken into use on a large scale. In this paper we report on the use over many years of process-modelling in connection to the quality system of Statoil, a large Norwegian oil-company, in particular on the aspects found necessary to emphasis to achieve the right quality of the models in this organisation. The Statoil-guidelines for enterprise structure and use of standard notation are mapped to the levels of SEQUAL, a generic framework for understanding the quality of models. Guidelines for modelling are found on most levels. More detailed guidelines than in general work on quality of business process models are found in particular on the physical, empirical, and syntactic level, where the number of detailed guidelines in Statoil has increased over the years due to needs identified.