The present paper deals with a specific usage of the term Ενέργεια in St Mark of Ephesus‘ treatise Capita Syllogistica. The text – probably composed during the Council of Ferrara-Florence or a short time later – in its essence summarizes the triadological argumentation of the Eastern participants in the council. The main question is addressed in the sense in which St Mark used the term Ενέργεια – as a predicate of person or of the essence, in the light of the monothelite and palamite controversies. The analysis of the paragraphs of the Cappita and the juxtaposition of the use of Ενέργεια in earlier sources lead to the conclusion that the transition of the term meaning from oikonomia to the explanation of the hypostatical relations and hypostatical idioms didn‘t question the ontological model of the relation God-energies-creation.