An in-line grading system, with near infrared (NIR) spectrometry and weigh-cell capability, was used to evaluate the total soluble solids (TSS) contents and fresh weights (FW) of all fruit from peach, nectarine and plum trees subjected to four fruit-thinning protocols. The grading platform achieved root mean squared errors of predictions for TSS of < 0.8%, and for FWs of 0.5 g.Thinning to one fruit cm–2 trunk cross-sectional area resulted in an increase in fruit TSS content of approx. 2% for peach and nectarine, and 0.5% TSS for plum, and an increase in fruit FW by an average of 30 g for nectarine, 40 g for peach, and 10 g for plum fruit. Fruits harvested from the upper tree canopy (> 3 m height) were higher in TSS contents compared to lower (0 – 1.5 m) canopy fruit (3% increase in TSS for peach, 2% for nectarine, and 1.5% for plum). Future work to introduce NIR technology into agronomic programmes aimed at improving the TSS contents of fruit is discussed.