The response of mass transfer to a small mass sink to hydrodynamic fluctuations in the concentration boundary layer has been calculated as a function of frequency. The dimensionless local flux was expressed as a series expansion of the dimensionless local diffusion layer thickness η and the dimensionless local characteristic frequency ξ in the low frequency range, and as the asymptotic power law $\xi^{-\frac{1}{3}}$, in the high frequency range. The two solutions were shown to overlap fairly well for 6 [les ] ξ [les ] 13. The overall transfer function over the whole mass sink area involves a spatial distribution for which the low-frequency approximation applies at the upstream end and the high-frequency approximation applies downstream. The average response at frequency f varies as f−1. These theoretical predictions were tested electrochemically by using a rotating disk. The modulated limiting diffusion current due to a fast redox reaction at small circular microelectrodes embedded in the disk was measured as a function of the frequency of the modulation of the disk angular velocity.