Formic acid (FA) is one kind of important bulk chemicals, which is recognized as a sustainable and ecofriendly energy carrier to transport H-2 via dehydrogenation or CO via decarbonylation. Expectantly, FA upon decomposition into H-2 and CO could be used as the syngas alternative for hydroformylation. In this paper, the behaviors of FA to release H-2 as well as CO following the distinct pathways were carefully investigated for the first time, and then established a new hydroformylation protocol free of syngas. It was found that the atmospheric hydroformylation of olefins with formic acid (FA) as syngas alternative was smoothly fulfilled over Xantphos (L1) modified Rh-catalyst under mild conditions (80 degrees C, Rh concentration 1 mol %, 14 h), resulting in >90% conversion of the olefins along with the high selectivity to the target aldehydes (>93%). By using FA as syngas source, the side-reaction of olefin-hydrogenation was greatly depressed. The in situ FT-IR and the high-pressure H-1 NMR spectroscopic analyses were applied to reveal how FA behaves dually as CO surrogate and hydrogen source over L1-Rh(acac)(CO)(2) catalytic system, based on which the deeply insight into the catalytic mechanism of hydroformylation of olefins with FA as syngas alternative was offered. (C) 2020 Elsevier Inc. All rights reserved.