The present study tried to phosphorylate rice protein (RP), a known insoluble food ingredient, and determine the improvement of its solubility. RP was allowed to react with sodium trimetaphosphate (STMP) at pH 11.5 and 35°C, and the results indicated that 20.6% of the RP seryl residues were phosphorylated. Interestingly, the solubility of phosphorylated RP (2.6%) was not improved compared with that of RP (2.5%) at pH 7. The involvement of hydrophobic interactions and disulfide bonds in phosphorylated RP solubility was further evaluated. The phosphorylation of RP in the presence of urea as a chaotropic agent for weakening the hydrophobic effect resulted in 22.0% phosphoseryl residues but still did not increase RP solubility. The reduction of RP disulfide bonds prior to phosphorylation resulted in 31.3% phosphoseryl residues and increased RP solubility to 8.3% at pH 7, indicating that disulfide bonds within RP could be responsible for the failure to increase its solubility after phosphorylation.