Petrophysical tests were conducted on core samples from the unsaturated zone of the Sherwood Sandstone Group in East Yorkshire. Tests were conducted to determine which physical parameters most influenced its hydraulic conductivity values. The main parameters analysed were grain-size distribution, pore-throat size distribution, clay content, mineralogy and porosity. A constant flow rate permeameter was used to measure saturated hydraulic conductivity values in the vertical direction (perpendicular to lamination), K_v, and horizontal direction (parallel to lamination), K_h. Hydraulic conductivity values in the vertical direction, K_v ranged from 0.004 to 0.12 m day⁻¹ while values in the horizontal direction, K_h, ranged from 0.01 to 0.17 m day⁻¹. Hydraulic conductivity anisotropy, K_h/K_v, varied from 0.6 to 35. Scanning electron microscope analysis showed this anisotropy to be caused mainly by millimetre-scale laminations. Representative bulk hydraulic conductivity values were estimated from the core data; bulk horizontal hydraulic conductivity, K_hb, was estimated as 0.1 m day⁻¹, and bulk vertical hydraulic conductivity, K_vb, as 0.01 m day⁻¹. Principal components analysis and multiple regression analysis were used to determine parameters that affect hydraulic conductivity most. Grain sorting is established to be the most important parameter to influence K_v values; samples with fine laminations have relatively low K_v.