Abstract: A hydroformylation system for making aldehydes includes: (a) a primary reactor provided with catalyst feed, syngas feed and olefin feed adapted to convert the olefin and syngas to product aldehyde; (b) a first liquid vapor separator coupled to the primary reactor for receiving output therefrom, adapted to separate the product aldehyde into a crude aldehyde product stream and a vent stream containing syngas and unreacted olefin; (c) a vent reactor coupled to the first liquid vapor separator to receive the vent stream therefrom, the vent reactor also being coupled to the primary reactor which is configured to provide catalyst thereto, wherein the vent reactor is operative to convert unreacted olefin in the vent stream from the first liquid vapor separator to additional product aldehyde.