An experimental study was conducted of coupled heat and mass transfer during unidirectional solidification of a binary solution with solute exclusion. This paper reports microscopic scale measurements of the two-dimensional solute concentration field for a freezing protocol that results in uniform cellular phase interface morphology. Densitometric analysis is used to evaluate solute concentration profiles in spatial and temporal coordinates for both the intercellular space and at the advancing liquid-solid interface along forward and lateral vectors. In conjunction with simultaneous temperature field measurements reported in a companion paper, the magnitude of constitutional supercooling is evaluated and compared with interface stability models available in the current literature.