The effect of two non-ionic chitosan derivatives (hydroxypropyl (HPCH) and hydroxyethyl (HECH) chitosans) and one ionic derivative (carboxymethylchitosan, CMCH) on the fresh-state properties of cement mortars was studied. Zeta potential measurements and particle size distribution were carried out in order to elucidate the action mechanism of the admixtures. Results were seen to be strongly dependent on substituents of the chitosan. Non-ionic derivatives had a weak dosage-related influence on the fresh-state properties. The ionic CMCH showed the more marked effect: it was found to act as a powerful thickener and to reduce the workable life of the fresh mixtures, whereas it caused a delay in the hydration of the cement particles. CMCH reduced the slump by 50% while commercial viscosity enhancers exhibiting larger molecular weights (hydroxypropylmethylcellulose, HPMC, and hydroxypropyl guaran, HPG) only reduced it by ca. 25%. The negative values of zeta-potential and the strong flocculating effect point to an adsorption of CMCH onto the positively charged cement particles. Optical microscopy and TEM observations showed the polymer giving rise to interlinking between cement particles.