A novel methodology is developed to visualize and quantify biomolecules adsorption at the cellulose film0liquid interface. Hydrogenated cellulose (HC) films were made from cellulose acetate and deuterated cellulose (DC) films produced using deuterated bacterial cellulose. Deuterated bacterial cellulose was obtained by growing the Gluconacetobacter xylinus strain ATCC 53524 in D2O media. Horse Radish Peroxidase (HRP), a robust and well knw enzyme, was selected as model functional biomacromolecule to adsorb at the cellulose interface. The film thickness and quantification of adsorbed HRP molecules were characterized by X-ray and neutron reflectivity (NR) measurements. Reflectivity data analysis reveals the cellulose films to be smooth (low roughness) and uniform. The HC and DC films are 206 A and 92 A thick, respectively, and both films swell in the aqueous buffer solution. In NR measurements, it is difficult to trace the adsorbed HRP layer on HC film due to the small scaterring length density (SLD) difference between HC and HRP providing no contrast. However, using deuterated cellulose (DC) film provides sufficient SLD difference (contrast) with respect to the SLD of HRP. The adsorbed HRP layer is 110A thick and occupies a volume fraction of 20%. Using deuterated cellulose films enabled the quantification of thin and partial layers of proteins at the liquid interface. Quantifying and controlling the morphology and functionality of biomolecules at the cellulose interface enables to efficiently develop and optimize low cost cellulose based diagnostics devices with superior functionalization.