Poissons ratio is the measure of the extent by which the lengthening of a rod induces its thinning and vice versa. Prismanes are the only molecular system that exhibits a negative Poissons ratio (Auxetic effect). On the basis of the Thorpe-Ingold effect, which underlines the auxetic effect in prismanes, it is expected that silicon-based prismane will manifest a smaller Poissons ratio than carbon-based prismanes and that fluorine substitution on the terminal rings will also diminish the effect. Stretching and compressing experiments show that indeed the auxetic effect is significantly reduced upon going from carbon to silicon and moreover that fluorine substitution reverts the behavior of silicon-based prismanes from showing a negative Poissons ratio to a positive one. This is probably the first example of a substituent-induced reversal of the mechanical property of a chemical system.