Αggregation of phenylacetylene macrocycles (PAMs) in solution has been studied by 1H NMR spectroscopy and vapor pressure osmometry. The association constant for dimerization, Kassoc, has been determined by curve fitting the concentration dependence of 1H NMR chemical shifts to a model for monomer−dimer equilibrium. The reliability of the NMR-determined aggregation constants and aggregate size have been independently verified by vapor pressure osmometry measurements. Thermodynamic parameters for association have been obtained from van't Hoff analyses which show the aggregation to be favored enthalpically. The aggregation of PAMs bearing various endo- and exo-annular functional groups and PAMs of different geometry and ring size has been studied. The type of pendant functional groups and the manner in which these groups are arranged on the macrocycle is shown to strongly influence self-association. PAMs substituted with electron withdrawing groups (e.g., esters) are more strongly associated than those beari...