In a previous bulletin of this series a study of the volatile aliphatic acids of corn silage was reported in detail. It was shown that acetic and propionic acids are normally present in considerable amount, while higher members of the series, as butyric, seemed to impart undesirable qualities, and their presence might be taken as evidence of incipient spoiling. The total volatile acid recovered by distillation did not, however, in any case account for all of the acidity observed by direct titration of the original silage juice. The difference represents therefore non-volatile acid, which heretofore has been assumed to consist principally of lactic acid. Since the non-volatile acid, as determined by this indirect method, was in most of our samples in excess of the volatile acid, it was considered of sufficient importance to warrant further study.