Pinot gris, Riesling, Cabernet franc and Cabernet Sauvignon from a vineyard in Virgil, Ontario, Canada were subjected by cluster thinning to two crop-level treatments (full crop, half crop), combined with three harvest dates (commercial harvest (T0), three weeks after T0 (T1), six weeks after T0 (T2)) in a randomised experiment in 2011 and 2012, with wines produced from all treatment replicates. We hypothesised that a full crop with substantially delayed harvest date might have a greater impact on wine aroma than reducing crop level. Wine aroma volatile analysis was carried out by GC-MS, with quantification by calibration with analytical standards prepared in model wine. Delayed harvest overcame the effects of crop reduction for almost all volatiles. Pinot gris and Riesling displayed increases in varietal aromas in the T1 and T2 wines, e.g. monoterpenes and norisoprenoids, and in esters, aldehydes and alcohols. Reduced concentrations of volatile acids and green odour compounds (e.g. 1-hexanol) with delayed harvest were also evident. Increases in ethanol were related to increased berry sugars, but higher alcohols did not necessarily increase with harvest date. Crop level had little impact on the sensory properties of these cultivars in both seasons. However, delayed harvest resulted in substantial sensorial changes. Due to these chemical and sensorial effects, delayed harvest is recommended, rather than crop-level reduction, to enhance wine quality in these cultivars. Extended harvest, e.g. T2 treatments, was associated with the production of benzaldehyde, diethyl acetal, and higher concentrations of higher alcohols, e.g. isoamyl alcohol and nonanol, which could be linked to pre-harvest desiccation. GC-MS sample preparation Aroma analysis by GC-MS was carried out in the 2011 and 2012 wines for the four grape cultivars based on Bowen and Reynolds (2012), with adjustments. A 30 mL sample was taken from each wine treatment replicate immediately prior to bottling, and was kept at 4°C in the presence of N 2 inert gas until analysis. In duplicate, 100 μL of an internal standard, prepared with 10 μL of 98% 1-dodecanol (Aldrich; Oakville, ON) in 10 mL of 100% ethyl alcohol (Commercial Alcohols, Brampton, ON), was poured into a 10 mL volumetric flask, followed by the addition of wine to the mark and mixed. The prepared sample was transferred into a 10 mL Gerstel extraction vial. A 10 mm stir bar ("Twister"; Gerstel, Baltimore, MD), coated with polydimethylsiloxane (0.5 mm film thickness), was added to the sample and it was stirred for 1 hr at 1 000 g for extraction at room temperature. After extraction. the stir bar was removed, rinsed with Milli-Q water (Millipore, Bedford, MA), and dried out with lint-free tissue, then placed in a 4 mL amber vial at 4°C until analysis on the same day. The stir bar was then inserted into the extraction glass tube inside the thermal desorption unit (TDU) attached to GC-MS. GC-MS conditions, and conditioning of material An Agilent 6890N/5975B GC-MS equipped with a Gerstel TDU, cooled inject...