With the TRIS experiment we have performed absolute measurements of the sky brightness in a sky circle at \delta = +42 ^ { \circ } at the frequencies \nu = 0.60 , 0.82 and 2.5 GHz .
In this paper we discuss the techniques used to separate the different contributions to the sky emission and give an evaluation of the absolute temperature of the Cosmic Microwave Background .
For the black-body temperature of the CMB we get : T _ { cmb } ^ { th } = ( 2.837 \pm 0.129 \pm 0.066 ) K at \nu = 0.60 GHz ; T _ { cmb } ^ { th } = ( 2.803 \pm 0.051 ^ { +0.430 } _ { -0.300 } ) K at \nu = 0.82 GHz ; T _ { cmb } ^ { th } = ( 2.516 \pm 0.139 \pm 0.284 ) K at \nu = 2.5 GHz .
The first error bar is statistic ( 1 \sigma ) while the second one is systematic .
These results represent a significant improvement with respect to the previous measurements .
We have also set new limits to the free-free distortions , -6.3 \times 10 ^ { -6 } < Y _ { ff } < 12.6 \times 10 ^ { -6 } , and slightly improved the Bose-Einstein upper limit , | \mu| < 6 \times 10 ^ { -5 } , both at 95 % confidence level .