After characterizations of the class L of selfdecomposable distributions on $${\mathbb{R}}^{d}$$ are recalled, the classes K p, α and L p, α with two continuous parameters 0 < p < ∞ and − ∞ < α < 2 satisfying $${K}_{1,0} = {L}_{1,0} = L$$ are introduced as extensions of the class L. They are defined as the classes of distributions of improper stochastic integrals ∫0 ∞ − f(s)dX s (ρ), where f(s) is an appropriate non-random function and X s (ρ) is a Lévy process on $${\mathbb{R}}^{d}$$ with distribution ρ at time 1. The description of the classes is given by characterization of their Lévy measures, using the notion of monotonicity of order p based on fractional integrals of measures, and in some cases by addition of the condition of zero mean or some weaker conditions that are newly introduced – having weak mean 0 or having weak mean 0 absolutely. The class L n, 0 for a positive integer n is the class of n times selfdecomposable distributions. Relations among the classes are studied. The limiting classes as p → ∞ are analyzed. The Thorin class T, the Goldie–Steutel–Bondesson class B, and the class L ∞ of completely selfdecomposable distributions, which is the closure (with respect to convolution and weak convergence) of the class $$\mathfrak{S}$$ of all stable distributions, appear in this context. Some subclasses of the class L ∞ also appear. The theory of fractional integrals of measures is built. Many open questions are mentioned.