This study is concerned with the distorted perception of surface topography when both surface height and surface stiffness vary. Three psychophysical experiments were conducted using virtual surfaces rendered with a force-feedback device. In Exp. I, we found that the threshold for detecting a height difference between two adjacent planes was quite small (0.17 -0.63 mm) and decreased as surface stiffness increased (0.4-1.0 N/mm). In Exp. II, we tested our force constancy hypothesis which stated that users maintained constant penetration forces while exploring haptic virtual surfaces. Data collected during lateral stroking of surfaces of varying stiffness supported this hypothesis. In Exp. III, subjects stroked two surfaces with a surface height difference of 2 mm (well above the thresholds obtained in Exp. I) and with varying stiffness values. Our results showed that the relative stiffness of the two surfaces dramatically affected subjects' ability to discriminate the height of these surfaces. Our findings underscore the importance of understanding the interplay of haptic rendering parameters. Future work will focus on the development of compensation rules for ensuring perceptual accuracy of haptic virtual environments.