This article reports a method for surface modification of multi-walled carbon nanotubes (MWNTs) using a low-pressure capacitively coupled RF glow-discharge. Ar/C₂H₆ and Ar/C₂H₆/O₂ gaseous mixtures were used to produce non-polar (np-) and polar (p-) coatings, respectively, onto MWNTs. After 5 min of plasma treatment at 20 W and 20 torr, strongly hydrophobic and non-electrically conductive np-MWNTs were produced. The p-MWNTs were strongly hydrophilic and showed no measurable hydrophobic recovery 2 weeks after treatment. Aqueous suspensions of p-MWNTs remained stable and free of agglomerates after being boiled. The ζ-potential of p-MWNT nanofluids was −40.3 mV, indicating a highly stable dispersion.