Abstract.We reconsider the mean-field Potts model with q interacting and r non-interacting (invisible) states. The model was recently introduced to explain discrepancies between theoretical predictions and experimental observations of phase transitions in some systems where the Z q -symmetry is spontaneously broken. We analyse the marginal dimensions of the model, i.e., the value of r at which the order of the phase transition changes. In the q = 2 case, we determine that value to be r c = 3.65(5); there is a second-order phase transition there when r < r c and a first-order one at r > r c . We also analyse the region 1 ≤ q < 2 and show that the change from second to first order there is manifest through a new mechanism involving two marginal values of r. The q = 1 limit gives bond percolation and some intermediary values also have known physical realisations. Above the lower value r c1 , the order parameters exhibit discontinuities at temperaturet below a critical value t c . But, provided r > r c1 is small enough, this discontinuity does not appear at the phase transition, which is continuous and takes place at t c . The larger value r c2 marks the point at which the phase transition at t c changes from second to first order. Thus, for r c1 < r < r c2 , the transition at t c remains second order while the order parameter has a discontinuity att. As r increases further,t increases, bringing the discontinuity closer to t c . Finally, when r exceeds r c2 t coincides with t c and the phase transition becomes first order. This new mechanism indicates how the discontinuity characteristic of first order phase transitions emerges.