The objective of this study was to evaluate the influence of the aspect ratio and granular medium size on the hydraulic behaviour of horizontal subsurface flow constructed wetlands (SSFs) by means of tracer tests. In addition, the accuracy of non-ideal flow models (plug flow (PF) with dispersion and complete stirred tanks (CSTs) in series with delay) in describing this hydraulic behaviour was assessed. Experiments were carried out in three pairs of lined beds that received an equal and controlled flow of primary effluent, and that had an almost equal surface area (54–56m² each bed) with the following aspect ratios: 1:1, 1.5:1, and 2:1. The size of the granular medium of each bed within each pair also varied from coarse granitic gravel (D₆₀=10mm, C_u=1.6) to small granitic gravel (D₆₀=3.5mm, C_u=1.7). The water depth of all the beds was set to 0.5m. Tracer tests were conducted by a single-shot injection of a dissolution of KBr into the inlet tubes of the beds. During the experiment, the hydraulic loading rate was of approximately 36mm/day in all beds. The results indicate that one unit increase in the aspect ratio caused an increase of the normalized delay time of the tracer curve response by a factor of between 1.6 and 1.7, and a decrease in the dispersion number of approximately between 0.3 and 0.4 (depending on the size of the medium). The effect of the size of the medium on the hydraulic behaviour was not as clear as it was for the aspect ratio. This is because there was no clear relationship between the medium size and the normalized delay time, and because significant differences in the dispersion number were only found in two of the three pairs of beds. Of these two pairs, the beds with a fine medium showed a reduction in the dispersion number by a factor of 0.6 in comparison with the beds with a coarse medium. The CSTs in series with delay model represented the data more accurately than the PF with dispersion model (either when closed–closed or open–open boundary conditions were considered). This is because the former fits the asymmetry of the tracer response more accurately. The number of CSTs in series necessary for the model ranged approximately from 1 to 3. The main conclusion of this study is that the construction of a horizontal SSF with a higher aspect ratio and finer medium improves the hydraulic behaviour of the system by reducing internal dispersion.