Using Krein's theory of strings, we penalize here a large class of positively recurrent diffusions by an exponential function of their local time. After a brief study of the processes so penalized, we show that on this example the principle of penalization can be iterated, and that the family of probabilities we get forms a group. We conclude by an application to Bessel processes of dimension δ ∈ ]0, 2[ which are reflected at 1. Let P x and E x denote, respectively, the probability measure and the expectation associated with X when started from x ≥ 0. We assume that X is defined on the canonical space Ω := C(R + → R + ) (where R + := [0, +∞[), and we denote by (F t , t ≥ 0) its natural filtration, with F ∞ := t≥0 F t .Let us start by giving a definition of penalization (see also Theorem 3.1):Definition 1.1. Let (Γ t , t ≥ 0) be a measurable process taking positive values and such that 0 < E x [Γ t ] < ∞ for every t > 0 and every x ∈ I. We say that the process (Γ t , t ≥ 0) satisfies the penalization principle if there exists a probability measure Q x defined on (Ω, F ∞ ) such that