Alessandro Pizzo One-particle (improper) states in Nelson's massless model. (613K, postscript) ABSTRACT. In the one-particle sector of Nelson's massless model, the one-particle states are constructed for an arbitrarily small infrared cut-off in the interaction term of the Hamiltonian of the System. The performed method is a constructive one which exploits only regular perturbation theory, by a suitable iteration scheme. The disappearance of one-particle states is showed in the limit of no infrared regularization. Constructive features, as regularity in some parameters, are also inquired.