A process for producing a solvent for use in microcapsules for pressure-sensitive copying paper which comprises hydrogenating a hydrocarbon mixture with a boiling range of 290.degree. - 400.degree. C, which is obtained as a by-product in the decomposition of petroleum etc. and is containing polycyclic aromatic hydrocarbon compounds, at a temperature within a range of 100.degree. - 400.degree. C and in the presence of a hydrogenating catalyst capable of catalyzing nuclear hydrogenation so as to hydrogenate the aromatic rings of the compounds which contain at least three aromatic rings and are principal components of said polycyclic hydrocarbon compounds, leaving at least two aromatic rings therein intact.