The effects of low (0.067 cm(3)/100 g) and relatively high (0.19 and 0.27 cm(3)/100 g) initial melt hydrogen concentration, solidification processing conditions, and grain refining on the formation of hydrogen-induced gas porosity in Al-4.5 wt% Cu-1.4 wt% Mg alloy have been quantitatively investigated. The study was conducted with unidirectionally cooled laboratory-size ingots solidified at 0.2-37 K/s. An optical microscope-based image analyzer and precision density measurement based on the Archimedes' principle were used to quantify the characteristics of the hydrogen-induced porosity in the ingots. Predictably, increase in melt hydrogen concentration and decrease in solidification rate increased the amount of porosity and average pore size. However, the effect of solidification rate was greater at the very low melt hydrogen concentration (0.067 cm(3)/100 g). These results are consistent with reported effects of solidification rate and melt hydrogen content on porosity formation in other aluminum alloys. Addition of grain refiner slightly increased the amount of porosity and the average pore size, especially at solidification rates above 1 K/s.