We demonstrated a new type of integrated optical (I0) sensors, viz., output grating couplers on planar waveguides. They respond (1) with submonomolecular sensitivity to the adsorption of molecules out of a gaseous or liquid sample covering the waveguide in the grating region, and (2) to changes in the refractive index of the sample. The working principle of the output grating coupler sensors is reciprocal to that of the input grating coupler sensors demonstrated earlier by our group. The effects (1) and (2) change the effective refractive index N of a guided mode in the coupler region, and to this change aN the sensors respond with a change in direction of the outcoupled beam, which can be sensitively measured with a position-sensitive-detector (PSD). We also devised a method to measure the absolute value N of the effective refractive index in which the outcoupling angles of two counter-propagating modes, coupled into the waveguide from opposite endfaces, are measured with one PSD. In our experiments we used embossed surface relief gratings of 2442 lines/mm on planar about 150 nm thick Si0,-TiO, waveguiding films on glass substrates. The changes in the outcoupling angles of both the TE and TM modes were measured with two PSD's at wavelength A = 632.8 nm. First results are presen?ed on the sensor's response to changes in relative humidity of ambient air, and on its use as a refractometer. We plan to use output grating couplers, with chemo-responsive adlayers on the waveguides as (bio)chemical sensors.