We use methods from time-frequency analysis to study the structural and linear topological properties of the space 9 B 1ω of ultradistributions vanishing at infinity (with respect to a weight function ω). Particularly, we show the first structure theorem for 9 B 1ω under weaker hypotheses than were known so far. As an application, we determine the structure of the S-asymptotic behavior of ultradistributions.