They use a vegetable-based dye whose exact formula is kept secret, but we do know a few key details.

Quick Scoop: What they dye the Chicago River with

  • The river is dyed with a plant/vegetable-based powder dye, not a petroleum or heavy-chemical dye.
  • The powder itself is actually orange , but it turns the water bright green once it disperses in the river.
  • The specific recipe is a closely guarded secret held by Chicago Plumbers Local 130 and the parade organizers.
  • Only about 40 pounds of this dye are used to turn the stretch of river downtown into that neon emerald shade.
  • The modern formula replaced an older tracer dye (fluorescein) that was considered too harsh environmentally and was phased out in the 1960s.

Is it safe?

  • Organizers describe the current dye as biodegradable and non‑toxic, and environmental advocates pushed for the switch to the vegetable-based version decades ago.
  • Independent commentary and coverage consistently note that it is designed to be safe for the river and aquatic life, and it dissipates after a few hours.

Little bit of story

The tradition actually grew out of plumbers using tracer dyes to find illegal pollution in the river—someone realized if the dye could reveal leaks, it could also turn the whole river green for St. Patrick’s Day, and the city ran with the idea. Over time, pressure from environmentalists pushed Chicago to move away from the old fluorescein tracer and toward the secret vegetable- based formula that’s used today.

TL;DR:
When people ask “what do they dye the Chicago River with,” the best public answer is: a secret, vegetable-based orange powder dye that turns the water bright green, formulated to be biodegradable and non‑toxic, and used in relatively small amounts for the annual St. Patrick’s Day show.

Information gathered from public forums or data available on the internet and portrayed here.