A model for predicting the response of apples (Malus x domestica) to chemical thinning treatments was evaluated on ‘Gala’/M.7 trees in 2008. The model assumes that fruit growing at a rate less than 50% of the fastest growing fruit on the tree during the period between four and seven days after application of the thinner will abscise. The model accurately predicted fruit set in trees that did not receive a chemical thinner but slightly over-estimated fruit set following application of 100 mg/L 6-benzyladenine (6-BA) plus 1200 mg/L carbaryl when fruit were 10 mm in diameter. This overestimation was due to abscission of some fruit that were growing faster than the 50% threshold rather than to retention of fruit that were growing at a rate less than the threshold. These aberrant fruit were consistently among the largest at the first measurement date. Application of 600 mg/L carbaryl at petal fall resulted in aggressive early chemical thinning, but reduced the efficacy of a subsequent post bloom application of 100 mg/L 6-BA plus 1200 mg/L carbaryl applied when king fruit diameter was 10 mm. The aggressive early thinning resulting from application of 600 mg/L carbaryl at petal fall created many rapidly growing fruit that were presumably more resistant to the post bloom application of 6-BA plus carbaryl.