A new type of aggregation behavior of block ionomers, i.e. functionalized polystyrene-block-poly[ethylene-co-(l-butene)]-block (SEES) with respective contents of 3.2 mol-% and 8.3 mol-% sodium carboxylate groups in polystyrene blocks has been studied using steady-state fluorescence techniques. It is found that the block ionomers could form a stable dispersion in water although both the slightly ionized polystyrene block and the ethylene/1-butene block are hydrophobic. The intensity ratio of the first to third vibronic band (I-1/I-3) of pyrene varies from 1.8, characteristic of the aqueous medium, to about 0.8, characteristic of hydrophobic microenvironment when the ionomer concentration in water is increased from 8x10(-4) g/L to 2x10(-1) g/L. Meanwhile, the emission intensity ratio of excimer to monomer (I-e/I-m) of pyrene increases with increasing polymer concentration first, then passes through a maximum and eventually falls off to zero. Both of the methods show the same critical aggregation concentration (8X10(-4) g/L). Besides the non-radiative energy transfer efficiency between naphthalene and anthracene shows a considerable increase when the concentration reaches 0.01 g/L. All of these three approaches lead to the fact that the block ionomers could form hydrophobic aggregates in water stabilized by the rare ionic groups.