Novel stable carboxylic acid derivatized alkoxy silanes are co-condensed with tetraethyl orthosilicate in the presence of a structure-directing agent in a direct ''onepot'' synthesis to porous carboxy-modified silica particles. Special emphasis is given to the influence of an increasing amount of 5-(triethoxysilyl)pentanoic acid and sodium hydroxide on the morphology and porous structure of the co-condensation products. The simultaneous conversion of both silanes resulted in the formation of mesoporous, monodisperse spherical particles with diameters between 100 and 140 nm and specific surface areas up to 900 m 2 g -1 . Additionally, adsorption equilibrium studies were performed, in which the particles modified with carboxylic acid groups showed an increased adsorption rate toward methylene blue compared to pure silica materials. Graphical Abstract