A high speed two-dimensional surface photovoltage (SPV) sensing system based on a digital data processing was developed and applied to an in-situ monitoring of chemical images. The SPV signal generated by a scanning light spot was directly memorized in a computer and signal integration of all measurement points was carried out in parallel in allocated memories by numerical calculation, which made it possible in principle to reduce the measurement time as short as the scanning time of the light spot. For the formation of the image of 6400 data points, the proposed system needs about 30 min which is at least one order of magnitude faster than that of a conventional analog SPV system.