A detailed investigation of the formation and properties of mesoporous silica templated by the chiral nematic liquid crystal phase of cellulose nanocrystals (CNCs) is presented. Under appropriate conditions, CNCs co-assemble with silica up to loadings of approximate to 60 wt% to give composite films with periodic chiral nanostructures. The periodicity of these films can be readily controlled to obtain materials that selectively reflect light with wavelengths ranging from approximate to 400-1400 nm. The co-assembly of CNCs and silica into ordered chiral nematic structures is demonstrated to occur within a narrow window of pH and is affected by aging: a slow rate of silica condensation appears to be vital for the formation of well-ordered materials. CNCs can be removed from the composite films by calcination or acid hydrolysis to give high surface area chiral nematic mesoporous silica (CNMS) with tunable pore diameters. The combination of mesoporosity and chiral nematic ordering in CNMS enables it to be used in a unique way for refractometric sensing applications. It is shown that, when using circular dichroism (CD) signals to monitor the chiral photonic properties of CNMS, variations in refractive index can be detected based on changes of both CD signal intensity and peak position with good sensitivity.