We report an investigation on the effects of counterions of magnesium salts on the homogeneous phase precipitation reaction to yield nanoparticles. The results provide new parameters to guide the control of the size, monodispersity, crystallinity, and morphology of Mg(OH)2 nanoparticles. Evidence emerged that magnesium counterions affect the size of crystallites. The particle size increases from about 50 to 200 nm and follows the Hofmeister anion series: sulfate < chloride < nitrate <= perchlorate. All of the results show that a proper choice of counterion and composition of reaction mixture allows one to modulate and set conditions that predetermine production of nanoparticles with desired size. The Mg(OH)2 nanoparticles have been applied to the successful conservation of paper. To demonstrate the efficacy of nanoparticles as deacidifying agent and as protection against cellulose aging, some samples of paper from the 18th century were treated with the nanoparticles and artificially aged by subjecting them to hydrothermal and photooxidative degradation. The treatment with nanoparticles preserved the mechanical features of paper, and the deterioration was consistently reduced. The comparison with the Wei t'O method (based on magnesium alkoxides), one of the most common and largely used methods for paper deacidification, indicates the following: (1) nanoparticles present a higher efficacy in the deacidification treatment since they are much more reactive; (2) nanoparticles are less aggressive since they are easily converted into the carbonate form; (3) they present minor disadvantages related to the chemical nature of the solvent used in the Wei t'O (i.e., CFC); (4) papers/books can be treated with very simple procedures, and do not require any special apparatus; (5) nanoparticle treatments have substantial economic benefits.