A series of brewing trials with hops of varying ages indicates that conductometric analysis is a reliable guide to the brewing value of a hop and that bitterness as estimated by the procedure of Brenner et al. correlates with statistically evaluated tasting. The ratio of the a acid content of a hop as estimated by conductometric titration with lead acetate to that estimated by polarimetry is a guide to its deterioration and behaviour during brewing. By the technique of thin-layer chromatography, 18 fractions have been observed in extracts of old hops and 22 fractions in bitterness extracts from beers brewed with such hops. Polarimetry is shown to be specific for the a acids, which may contribute as little as 20% of the total bitterness in beers brewed with old hops.