Research and development at Heraeus continued apace in the 20th century. One of the most outstanding technological innovations of the first half of the 20th century came from Heraeus: the process of melting metal materials under vacuum. For a time, Heraeus even operated its vacuum melting facility as an independent company that stood out for its innovative advances in the field. Between 1923 and 1933, the company received a total of 84 German patents.

In the 1920s, the third generation of the family joined the company. Dr. Wilhelm Heinrich Heraeus was responsible for technical management for almost four decades, while his cousin Dr. Reinhard Heraeus managed the business side of the company.

World War I, inflation and the collapse of markets brought Heraeus its share of difficulties, but the enterprise continued to grow. In 1939, 1,000 people worked for the company; its annual revenues had reached 20 million German marks. With Dr. Reinhard Heraeus at the helm, the family-owned company had finally developed into a multiproduct enterprise.

When World War II broke out, this development came to an abrupt halt. Many employees were drafted into the military, and bombing raids all but destroyed the production facilities in Hanau.