Little is currently known about the chemical changes that birch logs undergo during soaking in the plywood process. In order to probe the chemical changes in the log during soaking, sap obtained by mechanical compression from the log was chemically analyzed after pilot scale soaking at 70 °C. The results from the experiments revealed that the physical interaction between the core part of the log and the soaking water is limited. It was also suggested a water extraction causing significant increases in free monosaccharides and organic carbon concentrations in the sap. Assessment of the soaking water in the tank after soaking showed that the level of organic loading in water increases as the soaking temperature increases, probably due to the elution of organic substances from the bark. The dominant-free monosaccharides were glucose and fructose. Xylose and galactose also emerged during soaking. From the results, it is suspected that the majority of organic loadings in the water after soaking originate from the bark. This methodology for the sap collection seemed a reasonable approach to analyze the chemical characteristics of the xylem part of a log. Another aspect, relating to the seasonal variation of these changes in birch logs, is briefly discussed.