We study regularity properties of frequency measures arising from random substitutions, which are a generalisation of (deterministic) substitutions where the substituted image of each letter is chosen independently from a fixed finite set. In particular, for a natural class of such measures, we derive a closed-form analytic formula for the L q -spectrum and prove that the multifractal formalism holds. This provides an interesting new class of measures satisfying the multifractal formalism. More generally, we establish results concerning the L q -spectrum of a broad class of frequency measures. We introduce a new notion called the inflation word L q -spectrum of a random substitution and show that this coincides with the L q -spectrum of the corresponding frequency measure for all q ≥ 0. As an application, we obtain closedform formulas under separation conditions and recover known results for topological and measure theoretic entropy. CONTENTS 1. Introduction 1.1. Entropy and L q -spectra 1.2. Random substitutions 1.3. Statement and discussion of main results 1.4. Discussion and further work 2. Preliminaries 2.1. Symbolic notation 2.2. Dynamics, entropy and dimension 2.3. L q -spectra and smoothness 2.4. Multifractal spectrum and multifractal formalism 2.5. Random substitutions and frequency measures 2.6. Primitive random substitutions 2.7. Compatible random substitutions 2.8. Frequency measures 2.9. Separation conditions and recognisability 3. L q -spectra of frequency measures 3.1. Inflation word L q -spectra 3.2. L q -spectra for non-negative q