Chemical trapping of water and terminal OH groups in nonionic micelles of hexaoxyethylene monododecyl ether, C(12)E(6), by an aryl cation formed by spontaneous decomposition of an aggregate-bound 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, demonstrates that the interfacial region of a C(12)E(6) micelles is "wet" and that hydration of interfacial ethylene oxide groups depends on both surfactant concentration and temperature. Product yields are used to estimate hydration numbers of C(12)E(6) micelles by assuming that the selectivity of the reaction in micelles is the same as that of its short chain analog 2,4,6-trimethylbenzenediazonium ion, 1-ArN2+, in aqueous oligooxyethylene glycol solutions. The hydration numbers are found to decrease gradually with increasing C(12)E(6) concentration at 40 degrees C from 3.5 at 0.45% (0.01 M) C(12)E(6) to 2.5 in 54% C(12)E(6),just before the lamellar phase region, and to 0.84 in 82.5% C(12)E(6), above the lamellar phase region. The hydration numbers also decrease linearly with increasing temperature in 0.01 M C(12)E(6) from 4.2 at 20 degrees C to 2.9 at 60 degrees C, passing through the cloud point at 50 degrees C. The values of the hydration numbers are in good agreement with some estimates made from water self-diffusion measurements. Chemical trapping is a rapid, straightforward method for estimating hydration numbers of aggregates of nonionic surfactants that requires no information about their size and shape and that can be used in any fluid region of their phase diagrams. Potential applications are briefly discussed.