In a series of three papers we report the results of an extensive study on the magnetic properties of Cu(NO 3) 2.2 1 2 H 2O . In the first paper we will present the experimental data on the short-range ordered state. Magnetization isotherms as well as specific-heat and susceptibility experiments performed in external field are presented. The magnetic ions in this salt are coupled in pairs by an antiferromagnetic (af.) exchange interaction ( J/ k = -2.6 K. The pairs in turn are linked into linear chains by a much weaker antiferromagnetic interaction (-0.70 K). To describe the low-temperature thermodynamic properties in large external fields ( H ≈ 2β Jβ gβ) the pairs are represented by effective spins ( S′ = 1 2 ) in an effective field ( H E). The zero effective field specific-heat and susceptibility data are well described within this framework by theoretical curves for an af. linear chain with predominant X− Y exchange anisotropy. The field dependence of thermodynamic properties like the isothermal magnetization and the entropy, however, deviates from the predictions and reveals the shortcomings of the effective spin model. The origin of these discrepancies are discussed. From the angular dependence of the proton resonance spectrum the tensors of the proton-copper interactions are determined. The possible superexchange paths are discussed in relation to the crystal structure.