We present an analysis of the diffuse emission at 5 GHz in the first quadrant of the Galactic plane using two months of preliminary intensity data taken with the C-Band All Sky Survey ( C-BASS ) northern instrument at the Owens Valley Radio Observatory , California .

Combining C-BASS maps with ancillary data to make temperature-temperature plots we find synchrotron spectral indices of \beta = -2.65 \pm 0.05 between 0.408 GHz and 5 GHz and \beta = -2.72 \pm 0.09 between 1.420 GHz and 5 GHz for -10 \degr < |b| < -4 \degr , 20 \degr < l < 40 \degr .
Through the subtraction of a radio recombination line ( RRL ) free-free template we determine the synchrotron spectral index in the Galactic plane ( |b| < 4 \degr ) to be \beta = -2.56 \pm 0.07 between 0.408 GHz and 5 GHz , with a contribution of 53 \pm 8 per cent from free-free emission at 5 GHz .
These results are consistent with previous low frequency measurements in the Galactic plane .

By including C-BASS data in spectral fits we demonstrate the presence of anomalous microwave emission ( AME ) associated with the H ii complexes W43 , W44 and W47 near 30 GHz , at 4.4 \sigma , 3.1 \sigma and 2.5 \sigma respectively .
The CORNISH VLA 5 GHz source catalogue rules out the possibility that the excess emission detected around 30 GHz may be due to ultra-compact H ii regions .
Diffuse AME was also identified at a 4 \sigma level within 30 \degr < l < 40 \degr , -2 \degr < b < 2 \degr between 5 GHz and 22.8 GHz .