Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, commercially available as Poloxamers or Pluronics, are unique in forming ordered cubic phases consisting of reverse (water-in-oil) micelles. We set out to study the microstructure (form and dimension) as the reverse micelles order (from a micellar solution to a cubic lattice) with increasing block copolymer volume fraction and with increasing block copolymer molecular weight. The technique we used was small-angle neutron scattering (SANS) with solvent contrast variation. We selected four block copolymers with known phase behavior in water and p-xylene (Pluronics L44, L64, P84, and P104, all with the same PEO/PPO ratio and molecular formula (EO)(x)(PO)(y)(EO)(x), where x = 10, 13, 19, 27 and y = 23, 30, 43, 61, respectively) and worked in a dilution line with fixed water to copolymer content (1.2 mol of water per mol of EO). The temperature effect (22 and 45 degrees C) was also studied. The scattering behavior indicates that the micelles are approximately spherical but polydisperse. We used a two-sphere model where we assumed that all the PEO and the water are in the core of the micelle and that PPO forms a p-xylene-solvated shell. The micellar radius then depends on the molecular weight and the temperature and is approximately constant with concentration. The structure of the reverse micelles is also compared to that of normal (oil-in-water) micelles.