C-5 and C-6 unsaturated aldehydes are a category of BVOCs released by plants in response to their wounds and stress. In this work, the gas phase UV absorption spectra of trans-2-pentenal (t-2P), trans-2-hexenal (t-2H) and 2-methyl-2-pentenal (2M2P) are determined between 240 and 400 nm, at room temperature and at pressures (1-9) torr. The reported UV absorption cross sections are (cm(2)molecule(-1)sigma(t-2P,)( 330) = (4.86 +/- 0.53) x 10(-20), sigma(t-2H),( 330) = (5.43 +/- 0.57) x 10(-20) and sigma(2M2P,) (320) = (6.62 +/- 0.50) x 10(-20). Their photolysis constants as well as their atmospheric lifetimes toward photolysis were then estimated.