The trapping of dust grains in resonances with the planet Neptune can be affected by electromagnetic radiation. The interaction of solar radiation with a non-spherical particle depends on its size, shape, chemistry and orientation relative to the incident sunbeam. It has been shown earlier that the probability of resonant capture is increased if the transversal components of radiation pressure are close to 1 × 10−4. It is shown that some microphysical properties of dust particles captured in a mean-motion resonance with Neptune can be deduced based on the peak values of the radiation pressure components. In particular, it is demonstrated that chances for resonant capture are generally low for particles with sizes smaller than 4 μ m. This is due to increased optical sensitivity of these particles to the dominant wavelengths in the solar spectrum. The probability of resonant trapping also decreases when the refractive index of the particles decreases.