This paper delineates the influence of counterions on the aggregation behavior of 1-methyl-4-n-dodecylpyridinium surfactants, using conductometry and H-1-NMR spectroscopy. Three types of counterions have been studied : (i) halides, (ii) alkanesulfonates, and (iii) aromatic counterions. The critical. micelle concentration is found to decrease with increasing counterion size and increasing counterion hydrophobicity, whereas the degree of counterion binding increases. The aggregation behavior of 1-methyl-4-n-dodecylpyridinium surfactants with aromatic counterions is shown to be markedly dependent on the substituent (hydrophobicity) and the substitution pattern in the aromatic ring of the counterion. Depending on the molecular architecture of the aromatic counterion, extremely long wormlike micelles can be formed instead of(nearly) spherical micelles. NMR experiments revealed that all aromatic counterions intercalate in between the pyridinium headgroups of the micelles, with more or less the same degree of penetration. All results can be explained on the basis of counterion-surfactant and counterion-water interactions, taking into account the specific microenvironment in the Stern layer.