The reactions of acetone and cyclopentanone on a variety of zeolites with different acidities and pore sizes were investigated in detail by both in situ C-13 solid-state NMR and ex situ methods following extraction. The overall reaction sequence was acid catalyzed aldol condensations followed by secondary reactions such as double bond migration, hydrogen transfer, and cracking, especially in the more acidic zeolites. The formation of reactive complexes between the ketones and the Bronsted acid sites of the zeolites is implicated as a precursor to condensations at low temperature. The strength of complex formation was reflected in the extent of proton transfer to the ketone, and this was mapped into significant C-13 isotropic chemical shift changes. These are interpreted quantitatively as a measure of the effective acidity of the zeolitic environment under actual reaction conditions using a procedure proposed by Farcasiu and coworkers [J. Catal. 1992, 134, 118]. The order of activity for aldol condensations of acetone on the various zeolites (HZSM-5 > HY > HX > NaX, CsX > CsY, CsZSM-5) is in complete agreement with this in situ measurement of acidity. The zeolites studied here were not superacids at 298 K. Free carbenium ions or hydroxycarbenium ions did not form at 298 K as long-lived intermediates from the ketones studied, even at low loadings. The aldol products obtained from acetone and cyclopentanone were generally in agreement with solution chemistry with the exception that shape selectivity was evident in the formation of trindane from cyclopentanone on large pore but not on medium pore zeolites. Cyclohexanone formed products analogous to those of cyclopentanone. The in situ NMR experiments were effective guides to the design of zeolite-based syntheses of aldol products of cyclopentanone using either sealed glass tube or refluxing solvent protocols. The overall results of this investigation suggest the emergence of a physical organic pedagogy that will systematize synthetic reactions using zeolites. Such a methodology would be important in the development of zeolite-based procedures as alternatives to existing routes to fine chemicals that also produce corrosive liquid or metal salt wastes.