1. Introduction
2. The Sample
2.1. L1489-IRS
2.2. B5-IRS1
2.3. L1455-IRS1
2.4. L1551-IRS5
3. Observations
4. Results
4.1. Line Identification
4.2. Constraints on line optical depth
4.3. LTE Analysis
4.4. Line Profiles
4.5. Non-LTE Large Velocity Gradient (LVG) Analysis
5. Discussion
5.1. Chemical Diversity
5.2. The L1551-IRS5 system
5.3. Chemical Evolution Along the Star Formation Process
6. Summary