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[Display omitted] • Flexible hydroformylation of olefins using a cyclic monofluorophosphite ligand. • On-demand, in-flow aldehyde regioselectivity tuning from 90% linear to 75% branched. • Hydroformylation of 1-octene with a turnover frequency >75,000 mol ald.mol Rh−1.h−1. • Selective tuning of the ligand to metal coordination on-the-fly. • In-flow mechanistic studies of rhodium-catalyzed hydroformylation of 1-octene. Tuning aldehyde regioselectivity via homogeneous hydroformylation of olefins using the same catalyst system remains a challenge. Here, we present flexible rhodium (Rh)-catalyzed hydroformylation of 1-octene and propylene with a bulky cyclic monofluorophosphite ligand L. Hydroformylation of 1-octene with Rh/ L catalyst achieves, for the first time , turnover frequencies exceeding 75,000 mol ald.mol Rh−1.h−1 (at 30% conversion) in segmented flow, while enabling access to an unmatched tunable aldehyde branching (0.06 < linear/branched < 15) with the same ligand L. Our mechanistic studies demonstrate that L provides a viable alternative to traditional bidentate phosphine/phosphite ligands for high activity with the added benefit of tunable selectivity. The unique high flexibility feature of L over traditional linear- or branched-selective ligands allows for on-demand tuning from 90% linear to 75% branched aldehyde in a continuous flow reactor without the need for ligand/catalyst alteration. Furthermore, when starting from the internal olefins, Rh/ L catalyst achieves high regioselectivity (>90%) toward the two positional aldehyde isomers. The high turnover frequencies obtained with L in flow will enhance the economics of the production of aldehydes and their isotopically labeled analogues by significantly reducing the reaction time, thereby enabling better utilization of the increasingly expensive Rh catalyst and minimizing the need for catalyst/ligand separation and recycle. [ABSTRACT FROM AUTHOR]