A thermotropic nematic liquid crystal (LC) becomes an isotropic liquid at the nematic-isotropic transition temperature (T-NI), which depends on the molecular order of the mesophase. By means of a polarized optical microscope and a differential scanning calorimeter, it was found that doping the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (SCB) with molecularly rigid carboxylic acids (benzoic, 1-naphthoic, 2-naphthoic, and biphenyl-4-carboxylic acids) increases T-NI, without modification of the nematic-isotropic transition enthalpy. This increment in T-NI is due to the increased order caused by the formation of molecularly rigid and elongated dimers of carboxylic acids in the nematic LC, as confirmed with infrared spectra. Furthermore, T-NI, increased with the length of the molecularly rigid dimers at the same concentration level. Conversely, doping the LC with molecularly flexible acids caused lowering of T-NI. A quantitative correlation was established between the T-NI increase of the rigid carboxylic acids and the length of the dimers of these acids; a predictive model for these Delta T-NI, values as a function of acid molar fraction was developed. It was also demonstrated that the doping of 5CB with rigid carboxylic acids increases the rotational viscosity of the liquid crystal.