Support surface interface (footwear, flooring systems, etc), thickness, and hardness strongly influence stability in men of all ages. We hypothesize interfaces influence stability through their effect on proprioception. We tested this by means of an experiment based on a randomized, cross-over, and controlled comparison design. Footwear midsole hardness and thickness were independent variables. Dependent variables were foot position and perception of foot position, measured concurrently. Thirteen subjects were a random sample of healthy older men (mean age 72 years, sd ± 4.50). They were tested barefoot using six support surface interfaces consisting of shoes that were identical, except for midsole hardness and thickness, that spanned the respective ranges in current footwear. Measures were balance failure frequency defined as falls per 100 m of beam walking, rearfoot angle measured via an optical position measurement system, perceived maximum supination estimated by subjects via a ratio scale when walking, and foot position error, defined as rearfoot angle minus perceived maximum supination. The results demonstrated: (1) foot position awareness was positively related to stability; (2) foot position error was negatively related to support surface interface thickness; (3) foot position error was positively related to support surface interface hardness; and (4) foot position error correlated best with maximum supination. We conclude that instability induced by support surface interfaces is caused by its effect on foot position awareness. Thin hard-soled shoes provide superior stability for older men. Most currently available footwear provides poor stability because soles are too soft and thick. Since suboptimal support surface interfaces are encountered by everyone daily, they represent substantial safety hazards. Likewise, improving this situation through setting stability safety standards is a promising means of improving public safety. At the very least, the public must be informed about products imparting suboptimal stability so that they can anticipate potential problems and compensate for them without falling.