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The use of nanoparticle-in-matrix composites is a common motif among a broad range of nanoscience applications and is of particular interest to the thermal sciences community. To explore this morphological theme, we create crystalline inorganic composites with nanoparticle volume fractions ranging from 0 to ∼100% using solution-phase processing. We synthesize these composites by mixing colloidal CdSe nanocrystals and In 2 Se 3 metal–chalcogenide complex (MCC) precursor in the solution-phase and then thermally transform the MCC precursor into a crystalline In 2 Se 3 matrix. We find rich structural and chemical interactions between the CdSe nanocrystals and the In 2 Se 3 matrix, including alterations in In 2 Se 3 grain size and orientation as well as the formation of a ternary phase, CdIn 2 Se 4 . The average thermal conductivities of the 100% In 2 Se 3 and ∼100% CdSe composites are 0.32 and 0.53 W m −1 K −1 , respectively. These thermal conductivities are remarkably low for inorganic crystalline materials and are comparable to amorphous polymers. With the exception of the ∼100% CdSe samples, the thermal conductivities of these nanocomposites are insensitive to CdSe volume fraction and are ∼0.3 W m −1 K −1 in all cases. We attribute this insensitivity to competing effects that arise from structural morphology changes during composite formation. This insensitivity to CdSe volume fraction also suggests that very low thermal conductivities can be reliably achieved using this solution-phase route to nanocomposites.