This paper deals with the handling of implicit model formulations in MINLP optimization and proposes a strategy to cope with occurring difficulties of insufficient information. Process flowsheet simulators are the state-of-the-art tool of chemical engineers. Since these packages often utilize implicit model formulations for physical properties calculation etc., it is necessary to analyze the performance of MINLP algorithms when dealing with such problem representations. The examinations are carried out with the program system PSANO (Process Synthesis and Network Optimization), where the equation-oriented flowsheeting package SPEEDUP and the Mixed Integer Linear Programming (MILP) solver OSL have been integrated into an extension of the Outer Approximation (OA) approach.