Fruit thinning of peach and nectarine cultivars is essential to maximize profitability and stabilize production and is routinely done by hand thinning of blossoms and early fruit. Chemical thinning of blossoms may reduce the cost of manual labour, but results are inconsistent and commercialization is lacking. Blossom sensitivity to caustic agents may depend on blossom type and degree of pollination and/or fertilization. Armothin®, a registered blossom thinning agent, was evaluated at 1.5% v/v and 3% v/v applied at 11, 17 and 42% full-bloom, to evaluate the effect of blossom type and the fertilization and/or pollination of blossoms, on thinning of ‘Alpine’ nectarine. The earliest application (11% full-bloom) of 3% v/v Armothin® provided optimum thinning and fruit size. Any blossoms showing pink appear to be removed by Armothin®, while open blossoms are removed irrespective of whether they are fertilised and/or pollinated. The removal of early opening blossoms results in the potential loss in early ripening, higher value fruit especially in an early cultivar. Later maturing blossoms remain to develop into later maturing fruit which tend to be smaller. Thus an early application of 3% Armothin®, when less than 50% blossoms show pink, may provide sufficient blossom thinning to expedite later hand thinning, but does not necessarily remove the need for hand thinning.