Using a computerized energy compensated time of flight atom probe (APFIM), we have obtained the average composition (Si,H,O) of two widely differing hydrogenated amorphous silicon rf-sputtered films which were deposited onto W field emitter tips. Analysis of the evaporated ion species shows that the films prepared at the lower total gas pressure (5 mTorr) contain predominantly SiH and Si units while at the higher pressure (20 mTorr) the additional units of SiH2 and SiH3 are also present. These results are compared to films prepared and characterized on planar substrates and found to be in agreement.