Inactivation tests of Ascaris eggs (Ae) were performed using hydrogen peroxide and a Fenton type nanocatalyst supported on activated carbon (AC) (FeOx/C). Blank inactivation tests were also carried out using H2O2 and H2O2/AC as oxidation systems. The FeOx/C nanocatalyst was synthesized through a novel hybrid method developed in this work. The method is based on the incipient impregnation technique, using isopropyl alcohol as dissolvent and chelating agent of the iron salt and the ultrasonic method. The supported nanocatalyst contained 2.61% w/w of total iron and the support 0.2% w/w. Transmission electron microscopy (TEM)–energy dispersive spectrometer (EDS) images permitted verification of the presence of finely dispersed FeOx nanoparticles, with sizes ranging from 19 to 63 nm. SEM–EDS analysis and TEM images also showed good dispersion of iron oxide nanoparticles, most probably maghemite; γ-Fe2O3, able to produce hydroperoxyl radical as reported in the literature. The FeOx/C nanocatalyst-H2O2 system showed an average Ae inactivation efficiency of 4.46% Ae/mg H2O2. This value is significantly higher than the result obtained using the support-H2O2 system and H2O2 alone and it is also better than data reported for the classical Fenton process (homogeneous phase) with or without UV light.