The mechanisms that govern the ordered assembly of multiprotein ribonucleoprotein complexes are not well understood. The in vitro reconstitution of the small subunit of the bacterial ribosome provides a tractable system for the detailed study of ordered assembly. We present a quantitative thermodynamic description of the hierarchical binding of ribosomal proteins to 16S rRNA during assembly of the platform of the 30S ribosomal subunit. The binding of S8, S11, S15, and the S6:S18 heterodimer to the central domain of 16S rRNA has been measured both individually and in combination using isothermal titration calorimetry and gel mobility shift assays. Both enthalpy and free energy measurements demonstrate the cooperative binding of S15 and the S6:S18 heterodimer, but no cooperativity is observed for either S8 or S11. The results define a thermodynamic framework that describes cooperative platform assembly.