Many areas of research in Psychology are based on the implicit assumption that opposites lie at 2 extremes of the same continuum—e.g. semantic differential scales (Osgood, Suci, & Tannenbaum, 1957; Heise, 1970) and Likert scales (1932); just think, for example, of the use of “long” and “short” as parts of the same dimension ‘length’. The question of unidimensionality represents an interesting issue not only for methodology, but also, and mostly, represents a point of interest in the Psychology of Perception or Cognition. From the 1970’s onward, the study of opposites in Experimental Psychology has coincided, in particular, with the analysis of antonyms, shifting attention thereby from the empirical-perceptual foundations of this relation to linguistic rules. Savardi & Bianchi (1997, 2000, 2003, 2004a) have recently proposed an investigation on the perception of opposites that focuses on the phenomenal structures of experience, by starting from Gestalt assumptions concerning the direct perception of relations. Hence, it is within this frame of reference that the present study examined whether pairs of opposite properties do or do not lie on the same dimension. In 1967, Mosconi raised the issue of the fundamental difference that exists between phenomenal opposites (high and low, near and far, etc...) and the unidimensional structures that are used in their place in certain contexts (e.g., height and distance, with respect to the above examples). He maintained that these properties are not actually unidimensional in everyday language and phenomenal use. In a similar vein, Kennedy (2001) analyzed and interpreted the distribution of antonym adjectives in comparative linguistic constructions and proposed a model in which degrees of a property are considered scale intervals. There has long been a need (Campbell, 1920; Wright & Masters, 1982) to use psychometric instruments to validate the true dimensionality of a given construct. In the present work, we used a metrical measurement method, based on concrete objects, to examine whether the opposites of high-low, wide-narrow, long-short, and large-small would emerge as unidimensional scales (Luce, Krantz, Suppes, & Tversky, 1990).