Phospholipase D is an accepted tool in the transformation of phospholipids which is preferably performed in aqueous-organic emulsion systems. The hydrolysis of phosphatidylcholine and its transphosphatidylation with 1-butanol are extremely different in hexane and diethyl ether. Small amounts of primary ol secondary alcohols (<2%) are able to mediate the otherwise very slow reaction in hexane/buffer. The reaction systems were characterized by X-ray scattering, water content in the organic phases, and interfacial areas and tensions. The different packing density of the amphiphilic phosphatidylcholine molecules at the interface where the enzymatic reaction mainly takes place appears to be one of the major causes for the large solvent effects.