The relationship between the radial voidage profile, epsilon (r), and the radial distribution of centres of monosized spherical particles, f(r), is revised. A close expression is given for the section S(r, r(C)), the area of the segment of a cylindrical surface of radius r intersected by a particle centred at r(C) From this expression, epsilon (r) can be evaluated straightforwardly from the knowledge of f(r). The range 1 less than or equal to a less than or equal to 2 of aspect ratios (a = d(T)/d(P)) is then analysed specifically. For this range, the distribution of spherical particles has been characterized theoretically (Govindarao et at., 1992) and a simple expression allows the evaluation of epsilon (r). The use of this expression for actual catalyst particles, imperfect spheres showing a distribution of sizes, in finally analyzed.