At least 600 kilograms of propionic acid flowed into the Rhine on Friday evening when a tanker was being loaded in a port belonging to the chemical group BASF in Ludwigshafen.
The loading was then interrupted, as a spokeswoman for the company announced on Saturday.
The cause of the spill is not yet known and is now being investigated by the water police, a police spokesman said.
The leaked propionic acid is used in the manufacture of plastics and medicines and is flammable.
It can irritate the respiratory tract and cause skin burns and eye damage, it said.
BASF has therefore informed the responsible authorities.
The people living along the Rhine were also informed via the Rhineland-Palatinate Ministry of the Environment.
Propionic acid is readily biodegradable, the company spokeswoman said.
Due to the dilution, no danger to aquatic life can be assumed.