Generalized Nets (GNs) are used in the conceptual modelling of service systems. In the present paper, a GN model of a serial composition of services is proposed. The model includes intuitionistic fuzzy estimations of uncertainty in the form of intuitionistic fuzzy pairs. The intuitionistic fuzzy pairs are represented as characteristics of some tokens of the GN. A naming system for the places and the transitions of the GN is proposed which facilitates the analytical modelling. The proposed GN approach can be used in the construction of GNs representing more complex compositions of services. It can also be used to define new Quality of Service (QoS) indicators as a function of QoS indicators of the composed services.