Over the last twenty years, new methods have been researched and developed in order to obtain more eco-friendly chemical processes. Deep eutectic solvents and natural deep eutectic solvents have been introduced as promising alternatives for many applications. They have attracted significant attention from researchers looking for versatile solvents. In order for these solvents to eventually find a suitable application, it is important to characterize their physicochemical properties. This paper includes a theoretical overview of the physicochemical properties of eutectic solvents, their classification, preparation and application. Two natural deep eutectic solvents, prepared of choline chloride and citric acid (ChCl-Cit) and malic acid and glucose (Ma-Glu), were characterized in the experimental part. Viscosity, electrical conductivity, refractive index and pH were measured as a function of different temperatures (15–55 °C) and also different water contents in the mixture. A Walden plot showing the dependence of molar conductivity and viscosity was made from the obtained data. The densities were determined at 25 °C. As the water content increased, the density of the solvents decreased. All investigated solvents are Newtonian fluids and they have shown extreme acidity. They are quite viscous at lower temperatures and low water content. Viscosity and the refractive index have decreased, while electrical conductivity increased with increasing temperature and water content.