Formylation of morpholine with "supercritical'' CO2 using the bidentate ruthenium catalyst RuCl2(dppe)(2) affords high N-formylmorpholine production rate at almost 100% selectivity. The solventless reaction could be an interesting alternative to the present N-formylmorpholine synthesis routes, which are based on environmentally harmful formylation agents. Video monitoring of the reaction mixture during reaction revealed a complex phase behavior, including the formation of solid carbamate, which was most prominent at low conversion and gradually disappeared with higher conversion, i.e., with the formation of N-formylmorpholine and water as a by-product. Addition of water as an additive suppressed solid carbamate formation and enhanced the reaction rate. Infrared spectroscopy was applied to follow the changes in the liquid and dense gas phase. The ruthenium catalyst was confined to the liquid phase, containing morpholine, N-formylmorpholine, water, dissolved carbon dioxide and hydrogen. Although the solubility of the ruthenium catalyst in the liquid phase decreased with increasing conversion, good performance in the synthesis of N-formylmorpholine could be achieved.