Four field experiments were conducted from 2004 to 2006 in both New York State and one trial in Nueva Casas Grandes, Mexico. We evaluated several chemicals before bloom and at full bloom. Some of the chemical thinning agents we evaluated include soybean oil (8%) 15-35 days before bloom, ammonium thio-sulfate (ATS) (1-5%) at bloom, Lime sulfur (1-4%) plus Fish oil (2%) at bloom, Tergitol TMN-6, (0.5, 0.75%) at bloom, Entry (1-3%) at bloom, and Wilthin (0.5, .75%). Over the three years of our study ATS was the most consistent thinning agent but in 2006 it clearly over-thinned. Tergitol was also an effective thinner while Soybean oil before bloom, Lime sulfur and Entry were not effective. Our results show that peach chemical blossom thinners can reduce fruit set, improve fruit size, and improve the proportion of larger size fruits. However, in all cases a significant yield penalty or yield reduction was required to achieve large fruit size and this does not always translate to an improvement in crop value. Thinning beyond optimum crop load levels has a strong negative impact on yield and may reduce overall crop value. Our data indicate there is a optimum crop load each year to maximize crop value. The optimum is often different than growers assume when they hand thin. Our results suggest that chemical blossom thinning in peach could reduce the need for hand thinning significantly for peach growers. However, it is variable from year to year and while our data concludes that ATS is the most promising bloom thinner for peaches in New York State, this chemical is a potent thinner and can remove too many fruits for a commercial crop. Thus, to implement chemical blossom thinning in New York will require a method to determine the proper rate and/or timing to reduce the risk of over-thinning.