In human computer interaction readability of textual information is one of the foremost requirements for displays designed to provide visual information. In this context it is of main interest to identify the design attributes that influence readability, and investigate their relationship with measures of ease of reading. In this paper we present results from a readability experiment focused on text contrast that we performed on the Web. Our analysis indicate that light text on dark background is more difficult to read, and that the minimum luminance contrast between foreground and background color, in terms of CIELAB lightness difference, should be about 27 units.