Soot aggregates have a significant warming effect on climate, and their structural and optical properties may evolve in the presence of coatings. Here, the relative humidity (RH) dependence of soot aggregate restructuring induced by secondary organic aerosol (SOA) coatings was investigated in a series of photo-oxidation experiments. Burner-generated soot aggregates were classified by mobility diameter and injected into a smog chamber, where they were exposed to oxidation products of p-xylene; coated aggregates were subsequently conditioned at one of the following RHs: <12%, 20%, 40%, 60%, or 85%. Changes in diameter and mass were monitored using differential mobility and centrifugal particle mass analyzers, respectively. At RH < 12%, the SOA coating was too viscous to induce restructuring, so the particle diameter increased uniformly with coating mass. At RH ≥ 20%, the SOA coating induced restructuring, and the degree of restructuring increased with RH, indicating that the decreased viscosity and increase...