1. INTRODUCTION

2. HYGAL: KEY SCIENCE GOALS  
2.1. Probing the transition from diffuse atomic to molecular gas  
2.2. Probing the cosmic-ray ionization rate in the diffuse ISM  
2.3. Probing the chemistry of warm phases of the ISM  

3. OBSERVATIONS  
3.1. Source selection  
3.2. SOFIA observations  
3.3. Ancillary data sets  

4. FIRST RESULTS TOWARD W3(OH), W3 IRS5, AND NGC 7538 IRS1  
4.1. Spectra and line-of-sight properties  
4.2. Analysis  

5. DISCUSSION  
5.1. Column density ratios  
5.2. Variations along the sight line  
5.3. $^{12}C/{}^{13}C$ isotopic abundances  

6. SUMMARY