Magnetometer data was examined from the two Mariner 10 passages through Mercury's magnetosphere for evidence of Na+ electromagnetic ion cyclotron waves (ICW). These waves are expected to be produced if the thermalization of newly created Na+ pick-up ions occurs. We found no evidence of Na+ ICW. The spacecraft's rapid passage through Mercury's magnetosphere meant that the ambient magnetic field was only relatively constant for at most ∼3–4 Na+ ICW wave periods throughout each transversal, therefore limiting the spectral resolution of the Na+ cyclotron frequency. Our computations indicate that the wavelengths of Na+ ICW may be on the order of the system scale lengths. It is, therefore, questionable whether these waves can grow to sufficient amplitude to effectively thermalize these pick-up ions and incorporate them into Mercury's magnetosphere.