Ionization efficiency (IE) of different compounds in electrospray ionization (ESI) source differs widely, leading to widely differing sensitivities of ESI-MS to different analytes. An approach for quantifying ESI efficiencies (as logIE values) and setting up a self-consistent quantitative experimental ESI efficiency scale of organic compounds under predefined ionization conditions (ionization by monoprotonation) has been developed recently. Using this approach a logIE scale containing 62 compounds of different chemical nature and ranging for 6 orders of magnitude has been established. The scale is based on over 400 relative IE (DeltalogIE) measurements between more than 250 different pairs of compounds. To evaluate which molecular parameters contribute the most to the IE of a compound linear regression analysis logIE values and different molecular parameters were carried out. The two most influential parameters in predicting the IE in ESI source are the pK(a) and the molecular volume of the compound. This scale and the whole approach can be a tool for practicing liquid chromatographists and mass spectrometrists. It can be used in any mass-spectrometry laboratory and we encourage practitioners to characterize their analytes with the logIE values so that a broad knowledge base on electrospray ionization efficiencies of compounds would eventually develop.