Cellulose-phosphate composite membranes have been prepared from bacterial cellulose membranes (BC) and sodium polyphosphate solution. The structure and thermal behavior of the new composites were evaluated by X-ray diffraction (XRD), P-nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetry (TG) and thermomechanical analysis (TMA). From XRD analyses the Ialpha and Ibeta cellulose crystalline phases were identified together with crystalline sodium phosphate that covers the cellulose microfibrils as revealed by SEM. P NMR spectra show peaks assigned to Q and Q phosphate structures to be compared to the Q units that characterize the precursor polyphosphate. Glass transition temperature, T g, obtained from TMA curves and thermal stability obtained from TG and DSC measurements, were observed to be dependent on the phosphate content.