In comprehensive two-dimensional gas chromatography (GC x GC), samples experience two-dimensional separation implemented by a modulator which helps preserve the first-dimension separation and facilitates the second-dimension separation by periodically collecting, focusing and launching the material from the primary column onto the secondary column with a different stationary phase. Column overloading in GC x GC is a considerable problem, aggravated by the fact that two columns are involved. Broad first-dimension peaks of an analyte help produce smaller fractions of the analyte in the second-dimension, reducing the chance of secondary column overloading. One of the means to generate broad peaks in the first-dimension is to use thick film primary columns. A series of primary columns of various film thickness were tested in the study, and the results indicate that when other conditions are kept constant, 1 microm film columns often provide better resolution in both first and second-dimension but at the expense of a much longer separation time; 0.1 microm is clearly inadequate for GC x GC separation; 0.5 and 0.25 microm film columns seem to be the best compromises.