Static and dynamic light scattering and viscometry experiments were performed to study the phase behavior and association properties of a PEP-type (P and E represent poly(oxypropylene) and poly(oxyethylene), respectively) triblock copolymer, Pluronic R 17R4 [(PO)14(EO)24(PO)14], in aqueous solution. While at room temperature, 17R4 exists in the form of single coils even at fairly high concentrations, this copolymer does form micelles in aqueous solution at elevated temperatures, thus exhibiting a temperature-induced micellization behavior similar to that of the EPE-type triblock copolymers. To study the effect of the chain architecture, 17R4 and L64 [(EO)13(PO)30(EO)13] constitute a good pair of subjects for comparison, since they are very similar in chemical composition and are only slightly different in the total molecular weight. L64 forms micelles over large temperature and large concentration ranges, leaving a small comer for unimers at low temperatures and dilute concentrations. In strong contrast, 17R4 forms associated structures only at high copolymer concentrations within a narrow temperature region of a wedged shape. High critical micelle concentration but small aggregation number and small micellar radius are characteristics of 17R4 micelles formed in water. The entropy loss associated with the looping geometry of the middle poly(oxyethylene) block in micelle formation should be responsible for the largely reduced micellization ability of the 17R4 triblock copolymer in water. Interestingly, all the standard thermodynamic functions of micellization obtained for 17R4 are approximately only half as much as those for L64 in water.