The behaviour of ruthenium was studied in relation to the acidity of beta zeolite. The impregnation of beta zeolite with ruthenium chloride decreased the crystallite size of beta zeolite. The acidity of the beta zeolite influenced the particle size of ruthenium. The particle size of ruthenium increased with decreasing acidity. Even catalysts with large particle size of ruthenium were selective to hydrogenation of the C=C bond in cinnamaldehyde hydrogenation. The ruthenium catalyst with highest Bronsted acidity gave the highest conversion. The products formed were hydrocinnamaldehyde, cinnamaldehyde acetal and hydrocinnamaldehyde acetal; no cinnamyl alcohol was observed, which indicates protection of the C=O group during the hydrogenation via acetal formation.