In an attempt to prepare cellulose nanocrystals (CNCs) by TEMPO-mediated oxidation of microcrystalline cellulose (MCC), a new species of MCC called AaltoCell was placed under scrutiny. Unlike Avicel, AaltoCell is a never-dried species, still containing ca. 10% xylan in its structure. According to thermoporosimetry, the pore size distributions of AaltoCell and Avicel did not differ from each other but after TEMPO-oxidation, AaltoCell exhibited a far lower degree of porosity, hypothetically ascribed to its hemicellulose content. Surprisingly, the charge density imposed by TEMPO-oxidation did not vary between AaltoCell and Avicel despite the different microfibril widths observed, respectively. After TEMPO-oxidation and centrifugation the fine fraction share, indicating the CNC yield, was not more than 11% for AaltoCell, compared to 20% from Avicel.