Being downwind of eastern and midwestern industrial centers, the Hubbard Brook Experimental Forest offers a prime location to monitor long-term trends in atmospheric chemistry. Continuous measurements of precipitation chemistry during the last 10 yr provide a measure of recent changes in precipitation inputs of H ion. The weighted average pH of precipitation during 1964-65 to 1973-74 was 4.14, with a minimum annual value of 4.03 in 1970-71 and a maximum annual value of 4.21 in 1973-74. The sum of all cations except H ion decreased from 51 ~teq 1-1 in 1964-65 to 23 ~teq 1-1 in 1973-74 providing a significant drop in neutralizing capacity during this period. Based upon regression analysis, the input in equivalents of H ion and nitrate increased by 1. 4-fold and 2.3-fold respectively, from 1964-65 to 1973-74. Input of all other ions either decreased or showed no trend. Based upon a stoichiometric formation process in which a sea-salt, anionic component is subtracted from the total anions in precipitation, SO~-contribution to acidity dropped from 83% to 66%, whereas NOs increased from 15% to 30% during 1964-65 to 1973-74. The increased annual input of H ion at Hubbard Brook during the past 10 yr is highly correlated with the increased input of nitrate in precipitation.