Three new spin probes containing polyamine chains (2 or 3 nitrogen atoms) and nitroxide were synthesized. These compounds are stable in aqueous media at pH 5–10, and shape and intensity of their ESR spectra do not depend on pH in this range. The involvement of the spin probes in association in solution results in decrease of the spin mobility which appears as spectral anisotropy. The polyamine spin probes bind to siliceous nanoparticles in solution resulting in a reversible decrease in spectral intensity. These observations open a new way for monitoring silicic acid condensation. Sorption of the spin probes on solid silica gives rise to anisotropic spectra. The polyamine spin probes penetrate into growing cells of diatom algae in a similar manner to polyamine-containing fluorescent dyes.