Objectives: The study and use of marine and fresh water microalgae in therapy is a very recent phenomenon. Fresh water microalgae are used in food and to a lesser extent in cosmetics, however, not much is known about the applications of marine microalgae.   The Department of Applied Physics at the University of Vigo (Spain) has developed a marine microalgae culture system, and the species obtained are used to prepare products fit for thalassotherapy and spa treatments.   The aim of this study is to develop a procedure for designing, manufacturing and characterising spa products made from marine microalgae. Method: The growing interest in natural tested quality products for use in Spas has stimulated the search for product mixtures frequently used in Spas, such as clays and medicinal-mineral waters, with laboratory cultivated microalgae to guarantee quality and optimum growth.   A review of scientific literature on the applications of marine microalgae in skin care was the first stage in this study.   Marine microalgae are a source of vitamins, pigments, proteins and other beneficial substances for the skin (Spolaore et al, 2006). Only a few microalgae species are consolidated in the skin care market, wherein the principal ones are Chlorella and Spirulina (Stolz et al, 2005). Protein rich extracts of Spirulina have been used in cosmetics for preventing stretch marks and extracts from Chlorella vulgaris have been used to stimulate collagen synthesis in skin, thereby fostering tissue regeneration and reduction of wrinkles (Stolz et al, 2005). Other microalgae used are Nannochloropsis oculata and Dunaliella salina. Tests done with an extract of Nannochloropsis oculata have shown that this microalga acts as an optimum protective sheath against oxidative stress and positively influences collagen synthesis (Stolz et al, 2005). An ingredient of Nannochloropsis oculata has excellent skin elasticity properties (short and long-term) and was already in the market in 2005 (Stolz et al, 2005).   Microalgae culture has been on the rise in the past years. This is generally done in tanks or pools using a controlled culture medium and with either natural or fluorescent light that imitates the natural solar cycle (photobioreactors).   A species of Nanocloropsis was selected to study its use in Spas. This is an alga which can be cultivated using photobioreactors such that its growth and profitability can be controlled.   Mixtures with varying proportions of clays and mineral waters and seawaters were prepared.   Research equipment was used to analyse the behaviour of the mixtures. To that end, density, specific heat and thermal conductivity, which are important thermophysical properties, were used to assess these products for heat therapy. Results and Discussion: The preliminary studies have provided results on culture conditions of the microalgae. Results of heat behaviour were also collected using varying mixtures of clays and waters.   These results show that marine microalgae can be used in Spa treatments either in baths or as poultices, thus indicating that thermal retention capacity is improved upon mixture with clays, an aspect which is important for their use in heat therapy.