We explore the possible use of various momentum-space quantities as molecular descriptors in QSAR and QSPR studies. It is found that three-or four-descriptor models, that include molecular weight. provide useful correlations for it range of property and activity data, such as gas-chromatography retention times, gas-hexadecane partition coefficients and tadpole narcosis concentrations, The underlying reason for this success suggests the utility of hybrid models that take account also of the atomic character, albeit corrected for the number of bonds. (c) 2005 Elsevier B.V. All rights reserved.