In a project to investigate the chemistry of South African Plectranthus species, from the dichloromethane extract of the aerial parts of Plectranthus venteri, we isolated two known natural product acetophenones, namely 2-hydroxy-3,4,5,6-tetramethoxy-acetophenone (1) and 2-hydroxy-4,5,6-trimethoxy-acetophenone (2). Structures were assigned using NMR spectroscopy and HRTOFESIMS. Compound 1 was previously synthesised as a precursor to the polymethoxylated flavone nobiletin. The acetophenones exhibited remarkable inhibitory activities against the transfer of the IncW plasmid R7K in a bacterial plasmid transfer inhibition assay.