A miniature integrated optical refractometer chip based on grating coupling into a gradient effective index waveguide has been demonstrated. The gradient of the effective index resulted from shallow gradients of both the thickness and refractive index of a TiO2 waveguiding film deposited on a structured fused silica substrate. A method suitable for determining these gradients and data for the wavelengths of 633 nm and 785 nm is presented. Experiments have been performed where the test liquid was used as the cover medium of the waveguide, influencing the position of a laterally confined waveguide mode which represents a convenient on-chip measuring variable. Refractive indices determined by the chip are compared with those obtained by a commercial refractometer. The experimental results show a refractive index resolution of about 2 X 10-4 for this chip.