Vibrational relaxation of strontium monohydroxide (SrOH) molecules in collisions with helium (He) at 2 K is studied. We find the diffusion cross section of SrOH at 2.2 K to be &sgr; d = ( 5 ± 2 ) × 10 − 14 cm 2 ?> and the vibrational quenching cross section for the (100) Sr–O stretching mode to be &sgr; q ?> = ( 7 ± 2 ) × 10 − 17 cm 2 ?> . The resulting ratio γ 100 ?> = &sgr; d / &sgr; q ∼ 700 ?> is more than an order of magnitude smaller than for previously studied few-atom radicals (Au et al 2014 Phys. Rev. A 90 032703 ). We also determine the Franck–Condon factor for SrOH ( A ˜ 2 &Pgr; 1 / 2 ( 100 ) ← X ˜ 2 &Sgr; + ( 000 ) ?> ) to be ( 4.8 ± 0.8 ) × 10 − 2 ?> .