Funds under management rose 7% to £16.1bn in the quarter to December with £771mln of new funds and a £289mln gain from market movements.
Ian Simm, chief executive, said the drive generally towards greener and more sustainable economies and investments underpinned the improvement.
“The commitment of policy makers, the business community and wider civil society to combatting climate change, reducing pollution and forging a path towards a more sustainable economy has never been stronger. “
Broker Peel Hunt added that the inflows were split across a broad range of products with fixed income also seeing an inflow of £51mln, which represented a turnaround from outflows in previous periods.
The broker expects full-year assets under management to reach £17.5bn and raised its price target to 380p from 330p.but lowered its recommendation to 'add' due to the strong performance of the shares last year.
Impax more than doubled in value in 2019 and the shares stand at 387p currently.