Tri-generation systems are used to simultaneously produce electrical, heating, and cooling energy. These systems are usually more efficient than conventional systems for separate production and have smaller distribution losses since they are often located closer to the consumer. For achievement of the best technical and/or financial results, tri-generation plants have to be properly, i. e. optimally designed and operated. Operational optimization is used for short term production planning, control of tri-generation systems operation and as a part of design level optimization. In this paper an approach to operational optimization of tri-generation plants with reciprocating engines is presented with the following mathematical model. It is also explained how this algorithm might be embedded in some larger optimization procedure. In this approach, the importance of the part load performance of different units of the tri-generation systems is emphasized, especially of co-generation unit, i. e. engine generator set and thus it relies on manufacturers' data and is characterized with relatively high level of details examined. Mathematical model is based on the equipment performance based constraints and demand satisfaction based constraints with the possibility to add more equations if appropriate. Objective function for optimization is benefit-cost function. Optimal operation regimes for typical days for each month are obtained and analyzed. Impact of electrical energy price on pay-back period and primary energy saving is analyzed. Primary energy savings are determined and compared to maximal value that could be obtained.