Abstract: Conventional wisdom says that neutrino oscillations measure only mass-squared differences and not the absolute neutrino mass scale. This is true, however, only at leading order in the expansion parameters m_i/E, the ratios of the neutrino masses m_i (i = 1, 2, 3) to the neutrino energy E. At next-to-leading order, the oscillation phase includes terms proportional to m_i^4 − m_j^4 = Δm²_ij (m_i^2 + m_j^2), and is therefore sensitive to the absolute mass scale. I discuss the next-to-leading-order corrections using a wave-packet treatment and taking into account the neutrino-production kinematics. The result can be applied to reactor neutrinos and, it turns out, the JUNO experiment is sensitive to neutrino masses of a few hundred keV.