An in-line holographic microscope is used for measurements of micrometer-scale particles and associated suspending fluid medium containing the particles. The system yields heterodyne scattering patterns that may be interpreted with Lorenz-Mie theory to obtain precise time-resolved information on the refractive index of the suspending medium for determining chemical composition, concentrations and makeup thereof. This approach can perform spatially resolved refractometry with measurements on calibrated refractive index standards and monitor chemical concentration in a microfluidic channel. Using commercially available colloidal spheres as probe particles and a standard video camera for detection yields volumetric refractive index measurements with a resolution of 2×10−3 RIU for each probe particle in each holographic snapshot.