Factors influencing sizing response were investigated by the combined analysis of X-ray photoelectron spectroscopy (XPS) and pyrolysis gas chromatography (PyGC). Fatty acids (FAs) such as stearic acid (SA) and palmitic acid (PA) were used as models of rosin size, and the handsheets prepared beforehand with aluminum sulfate (alum) were soaked in the FAs⁄chloroform solution. Nearly no sizing effect appeared in the FAs-sized sheets without alum. The sizing degrees of the sized sheets increased with an increase in the internal alum addition level, although the size content determined by PyGC was almost constant regardless of the amount of alum added or a variety of FAs as sizing agents. However, the sizing response of the sheets sized by SA⁄PA mixture was quite higher than that of SA- or PA-sized sheets. High resolution XPS analysis revealed that both of internal alum addition and blending FAs brought about an increase in the atomic ratio of carbon originating from the size components present on the pulp fibers. As based on the XPS analytical principle that the detectable depth of photoelectrons is quite shallow (‹ 10 nm), it was possibly suggested that one of the factors affecting the sizing response was the subtle size aggregation over 10 nm in diameter on the sheet surfaces. A creative analysis of XPS⁄PyGC comparative quantification offered a new aspect in sizing mechanisms that the delicate behavior of size aggregation must govern the sizing response.