Abstract: A process for producing mixed poly(vinyl acetals) containing butyral acetal groups and long chain acetal groups derived from one or more aldehydes containing at least six C atoms, by conducting an acetalization reaction forming long chain acetal groups in the substantial of absence butyraldehyde until gel break occurs, adding butyraldehyde after gel break and continuing acetalization until the desired reaction endpoint is reached. Reaction temperature is from about 40.degree. C. up to the boiling point of water at the pressure of the acetalization reaction and preferably between about 75.degree. C. and about 98.degree..